I've been running one of these for years and when I switch to ITBs (very soon now) would it matter if I leave off the vacuum connection? TBH I am not really sure what it does anyway - fuel enrichment comes to mind

It is supposed to increase fuel delivery as vacuum decreases, if you have fuel injection fully controllable, then this may only fight against what the ecu wants to do.
Also your fuel pressure regulator at the end of the fuel rail wants to give constant fuel pressure

But I thought the FSE was just an adjustable FPR to raise the rail pressure - it is certainly the only FPR in my system anyway. I originally adjusted it to 2 bar pressure at tickover and haven't touched it since. Doesn't the OEM FPR have a vacuum connection?

*edit* I just found the info on the FSE website (duh!)
http://www.car-performance-parts.co.uk/performance_road_products/power_boost_valves/power_boost_valves.htm

Looks like it is simply an FPR with bells on, so leaving the vacuum pipe off shouldn't matter as long as the map is taking this into account.

[Edited on 26-11-10 by RazMan]