I recently swapped to 3 stage vtec couple of months back. Still have several issues to sort out.
You will only be able to get the best fuel economy out of it when every component is in working order. As for me, I am getting around 7-8km/Ltr. No I am not flooring the pedal, and 85% of the time drive under 2000rpm, while 10% 2000-2500rpm, and 5% maybe above 3,000rpm.
3 stage vtec engine is quite good, but a PITA swap unless you are able to get every component working. You will need the best electrician who knows this engine inside out for the wiring, who will charge you approx 10k (excluding any sensors or parts required to be changed).
The total costs involved will go above 100k.
You can go through my thread for information: https://www.pakwheels.com/forums/civic/250052-civic-95-engine-swap
You can consider going for D15b non-vtec as well (if you want to covert to EFI). It will be a little underpowered, but less complicated.