* With an ASIO wrapper like ASIO4ALL
from Michael Tippach, you can use VAC in ASIO supporting applications.
VAC3 can be better than VAC4 if both sides applications use the MME (waveIn/waveOut) audio interfaces and can be configured to use a single, common audio format. If at least one application uses DirectSound interface and/or different audio format, VAC4 is preferred.
In any case, you always can download a free trial and test both versions. VAC3 and VAC4 can coexist in the same system if you choose different installation directories and Start Menu groups. But different versions of VAC3 or VAC4 cannot coexist in the same system.