I have an ipv2 and a nemesis mod and ive got a vtc5 30a 2600mah and an efest 35a 2500mah, which battery should i use in which device for best performance?
It depends.
For starters, that Efest is actually a re-wrapped 20 amp LG battery, the 35 amps is a pulse rating.
The IPV2 needs a 30 amp battery, particularly when you're pushing the wattage up hard. It would
probably be okay to use the Efest if you're staying under ~30 watts or so but I'd use the Sony just to be on the safe side.
Now, as to the Nemesis, if you keep your builds above .21 ohms, you
should be "safe" with the Efest (You'll be right at 20 amps), but I personally prefer a little more of a safety buffer and would stay above .3 ohms (0.3 ohms at 4.2 volts gives you 14 amps).
Less than that, I'd only use the Sony.
In summary? VTC5 in the IPV2, Efest in the Nemesis, but only above .21 ohms - the further above, the better.