Extract feature vector of a CNN
In order to "tap" intermediate layers of an existing model you could do the following:

# get your feature layer
cnn_model = load_model('weights/pre-trained_CNN.hdf5')
feature_layer = cnn_model.get_layer('fc7')

# stack your LSTM and other layers below, e.g.:
lstm_layer = TimeDistributed(LSTM(...), input_shape = ...)(feature_layer)
output = Dense(...)(lstm_layer)

# create a combined model 
model = Model(inputs = cnn_model.input, outputs = [output])