Попытка добавить QLayout в QWidget, у которого уже есть макет

Размещение some_layout(some_widget) аналогично размещению some_widget.setLayout(some_layout), поэтому в каждом макете вы добавляете его в self.window.

Вдобавок MainWindow уже имеет макет по умолчанию, так как у него есть определенный виджет по умолчанию, такой как QStatusBar и QMenubar, поэтому вы не должны добавлять его таким образом, вы должны добавить его в centralWidget, в вашем случае это может быть self.window.

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Window(QMainWindow):
    def __init__(self):
        QMainWindow.__init__(self)
        widget = QWidget(self)
        self.setCentralWidget(widget)
        self.setWindowTitle('Network Automation')
        self.lb_device_list = QLabel('Device Type')
        self.cb_device_list = QComboBox()
        self.cb_device_list.addItem('cisco_ios')
        self.cb_device_list.addItem('cisco_s300')

        self.vbox = QVBoxLayout()
        self.vbox.addWidget(self.lb_device_list)
        self.vbox.addWidget(self.cb_device_list)

        self.layout = QGridLayout()
        self.layout.addLayout(self.vbox,0,0)
        widget.setLayout(self.layout)


if __name__ == "__main__":
    app = QApplication(sys.argv)
    myapp = Window()
    myapp.show()
    sys.exit(app.exec_())

Еще одно замечание: вы не должны наследовать от QMainWindow и QWidget за раз, поскольку QMainWindow наследуется от QWidget, в этом нет необходимости

Другой вариант - реализовать класс, который наследуется от QWidget, а не от QMainWindow.

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Window(QWidget):
    def __init__(self):
        QWidget.__init__(self)
        self.setWindowTitle('Network Automation')
        self.lb_device_list = QLabel('Device Type')
        self.cb_device_list = QComboBox()
        self.cb_device_list.addItem('cisco_ios')
        self.cb_device_list.addItem('cisco_s300')

        self.vbox = QVBoxLayout()
        self.vbox.addWidget(self.lb_device_list)
        self.vbox.addWidget(self.cb_device_list)

        self.layout = QGridLayout()
        self.layout.addLayout(self.vbox,0,0)
        self.setLayout(self.layout)


if __name__ == "__main__":
    app = QApplication(sys.argv)
    myapp = Window()
    myapp.show()
    sys.exit(app.exec_())

Я также преобразовал ваш реализованный код в класс, унаследованный от QWidget:

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Widget(QWidget):
    def __init__(self, parent=None):
        QWidget.__init__(self, parent=parent)
        self.device_list = []
        self.setWindowTitle("Network Automation")
        ############################# Input IP

        # Device Type
        lb_device_list = QLabel(self)
        lb_device_list.setText('Device Type')
        self.cb_device_list = QComboBox(self)
        self.cb_device_list.addItem('cisco_ios')
        self.cb_device_list.addItem('cisco_s300')

        lb_ip = QLabel(self)
        bt = QPushButton(self)
        btadd = QPushButton(self)

        # Ip Device
        lb_ip.setText('IP Address')
        self.le_ip = QLineEdit(self)
        self.le_ip.setText('')
        self.le_ip.setPlaceholderText('Input Device IP')
        self.le_ip.setFixedWidth(150)

        # username
        lb_username = QLabel(self)
        self.le_username = QLineEdit(self)
        lb_username.setText('Username')
        self.le_username.setText('')
        self.le_username.setPlaceholderText('Input Username')
        self.le_username.setFixedWidth(150)

        # password
        lb_password = QLabel(self)
        self.le_password = QLineEdit(self)
        lb_password.setText('Password')
        self.le_password.setText('')
        self.le_password.setPlaceholderText('Input Password')
        self.le_password.setFixedWidth(150)

        # Privilage Password
        lb_enable = QLabel(self)
        lb_enable.setText('Privilege Mode Password')
        self.le_enable = QLineEdit(self)
        self.le_enable.setText('')
        self.le_enable.setPlaceholderText('Input Enable Password')
        self.le_enable.setFixedWidth(150)

        bt.setText('Generate')
        bt.setFixedWidth(70)

        btadd.setText('Add')

        line = QFrame(self)
        line.setFrameShape(QFrame.VLine)
        line.setFrameShadow(QFrame.Sunken)
        line.setLineWidth(3)

        ########################### Layout Ip Device List

        lb3 = QLabel(self)
        lb3.setText('IP Device List')
        self.combobox_ip_list = QComboBox(self)
        self.combobox_ip_list.setFixedWidth(170)
        #combobox_ip_list.addItems(ip_list)

        ############################## SubLayout and Layout
        hblayout = QHBoxLayout()
        hblayout.addWidget(bt)
        hblayout.addWidget(btadd)

        sublayout = QVBoxLayout()
        sublayout.addWidget(lb_device_list)
        sublayout.addWidget(self.cb_device_list)
        sublayout.addWidget(lb_ip)
        sublayout.addWidget(self.le_ip)
        sublayout.addWidget(lb_username)
        sublayout.addWidget(self.le_username)
        sublayout.addWidget(lb_password)
        sublayout.addWidget(self.le_password)
        sublayout.addWidget(lb_enable)
        sublayout.addWidget(self.le_enable)
        sublayout.addLayout(hblayout)

        sublayout2 = QVBoxLayout()
        sublayout2.addWidget(lb3)
        sublayout2.addWidget(self.combobox_ip_list)
        sublayout2.addStretch(1)

        layout = QGridLayout(self)
        layout.addLayout(sublayout,0,0)
        layout.addWidget(line,0,1)
        layout.addLayout(sublayout2,0,2)

        btadd.clicked.connect(self.addDevice)

    def addDevice(self):
        device_type = str(self.cb_device_list.currentText())
        ip = self.le_ip.text()
        username = self.le_username.text()
        password = self.le_password.text()
        secret = self.le_enable.text()
        command = 'show tech'
        self.device_list.append(MyClass(device_type, ip, username, password, secret, command))
        #ip_list.append(ip)
        self.combobox_ip_list.addItem(ip)# Add Ip to ComboBox

        for list in self.device_list:
            print(list.device_type)

class MyClass(object):
    def __init__(self, device_type=None, ip=None, username=None, password=None, secret=None, command=None):
        self.device_type = device_type
        self.ip = ip
        self.username = username
        self.password = password
        self.secret = secret
        self.command = command
#ip_list = []



##################################
app = QApplication(sys.argv)
QApplication.processEvents()
app.setStyle('cleanlooks')
window = Widget()
window.show()
sys.exit(app.exec_())