The first question to ask yourself is: why use an instance of the VClamp class to do this? In the Programmer's Reference, note that VClamp
https://nrn.readthedocs.io/en/latest/py ... tml#VClamp is described as
Two electrode voltage clamp.
Last time I saw anyone using a two electrode voltage clamp, they were using sharp glass microelectrodes to do experiments on muscle cells or the cell bodies of neurons in big invertebrates like Aplysia californica. Also, because of the technical problems involved in such experiments, VClamp comes with a lot of extra complexity (actually emulates some of the properties of the electrode preamp and the feedback amplifier).
None of that is relevant to patch clamp. If your models involve neurons that are typically studied with patch clamp, you should probably use an instance of the the SEClamp class.
Next, your question:
I am trying to do the trivial task of adding a depolarization step during pyramidal cell voltage clamp.
The holding potential is at -70mV and after some time I want to depolarize the cell to 0mV for 200ms. I treat this as two separate voltage clamps.
Easy. Create a single instance of the SEClamp class that is attached to the segment you want to clamp, and use its amp* and dur* parameters to specify what it does. SEClamp implements the classical "three step" protocol, in which
from t==0 to dur1 the command potential is amp1
from t==dur1 to dur1+dur2 the command potential is amp2
from t==dur1+dur2 to dur1+dur2+dur3 the command potential is amp3
for t>dur1+dur2+dur3 the clamp is off (delivers no current to the cell)
SEClamp's rs parameter allows you to specify the value of a series resistance (like the series resistance of a patch electrode).