I think one of the tricky things is that Serph's been pretty avoidant and cagey about the fact he's been eating monsters. (Just kinda letting people assume that he's been eating at the dining car.)

So I think in order for Serph to agree to that, V will need to demonstrate to him something like... wanting to try eating monster meat or something :V (And even then, Serph won't outright say that he's been doing that all this time.)