What is the difference (practically) between different types of rennet?
From what I've found it's pretty much a one to one ratio, if you are subbing vegetarian rennet for anmial rennet.  They act simmilarry in equal doses, so you should be fine just following the instructions for calf's rennet.  

I do perfer using calf's rennet over vegetarian, but that may be just be me being anti-vegetarian.