Ben,
BizTalk operates in near real-time so long as the load on the message box isn’t too high – and by real-time, I mean things should happen straight away, there shouldn’t be any delays. So, if VHI passes the message back to BizTalk, it will operate on it straight away.
I’ve not come across the VHI Adapter you’re talking about here – can you provide some more information on whether this is a custom adapter, or does it interface through the standard SOAP/HTTP/Other out-of-the-box BizTalk adapter?
If you are concerned about performance and you are using orchestrations, you could consider adding System.Trace statements to aid debugging to see whether the return message is being operated on immediately. Also, the BizTalk Admin Console/HAT should give you a good idea of message states.
The community should be able to provide more information with a little more information.
Cheers, Nick.