When I built mine up with a 65mm shock, I took the valve core out and fully compressed the rear suspension with the shock installed to verify clearance. Fully compressed there looked to be ~15-20mm of clearance from the seatstay bridge to the back of the seat tube. Way more clearance between the reservoir and downtube. I am running the RockShox C34 tune on my Vivid (which is the generic retail tune) as opposed to the next firmer C37 tune that SC usually specs, and am able to run the HSC in the middle setting instead of -1 like I do on my other bikes' C37 tune shocks. I am also running less sag with the longer shock, equivalent to as if I was still running a 60mm shock, so about 27% on the 65mm. This keeps the bike's geometry at sag feeling balanced - I started running 30% and the rear felt too mushy relative to the front. Running at 27% makes it still responsive pedaling and pumping but gives extra cushion on big hits.
I haven't tried the low progression setting on the Vala, went straight to high. I've tried low/high on other SC bikes and usually the less progressive setting feels smoother and more consistent near sag, and the higher progression setting feels more active off the top. From tinkering on other bikes, I know that I prefer to run very progressive bike frames with fewer volume tokens in the shock (and also prefer large volume air shocks to coil). Anyways, try both and see what you like better, keeping in mind you will probably find different pressures/spring rates work better for you in each setting.