Quick but hacky answer for your case, select target layer as:
model[0][0][-1]
Alternate answer:
This way of initializing the model is not good. Have a look over here regarding how to add extra layers to your model:
Regarding gradcam, you need to define your target layer. For that, you can check what is the target layer for Vgg:
vgg.features[-1]
MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
So, in your new model, it should refer to this layer as well. For getting that layer just try to print your custom model and try to find the layer