Why did you go with a 38mm?

Spring size would depend on how hard you want to push the VF22 (and what color/size spring came with it). Taking into consideration that you're running meth...I'd go for a combo that will give you a calculated rate somewhat close to 17psi. Remember that actual opening will be around 1-2psi less. A 17psi combo will keep it from cracking open while you're just cruising around yet still give you plenty of headroom for WGDC% when tuning for full boost.