the primary reason a VAFC exists is to tune A/F ratio. changing vtec engagement is also a feature of the unit but it's not as though you can set it at 2000 rpm and "Be iN VtEc AlL ThE TiME y0!"

when you change vtec x-over you do it to flatten out the hp/tq curves. instead of having a dyno plot that looks like the grandcanyon, you can tune A/F and VTEC x-over so you have a smooth set of curves and smoother power delivery. tuning for this kind of result takes some effort so yes get a VAFC and YES get it dyno tuned.