Rewrapping the modules in an nn.Sequential block can easily break, since you would miss all functional API calls from the original forward method and will thus only work if the layers are initialized and executed sequentially.
For VGG11 you would be missing the torch.flatten operation from here 2, which would create the shape mismatch.
The safest way would be to create a custom model, add the custom layer, and override the forward.
Alternatively, you could also replace an already used layer (e.g. the last nn.MaxPool2d layer) with an nn.Sequential container containing itself and the new custom layer.