To do so, you need to modify VGG.features. It is a nn.Sequential object and so all you need to do is modify the layers this specific object, you don’t need to modify the forward. Something of the following:
model = torchvision.model.vgg19(pretrained=True)
feats_list = list(model.features)
new_feats_list = []
for feat in feats_list:
    new_feats_list.append(feat)
    if isinstance(feat, nn.Conv2d):
        new_feats_list.append(nn.Dropout(p=0.5, inplace=True))

# modify convolution layers
model.features = nn.Sequential(*new_feats_list)