Here's a link to our earlier thread about vinyl pools and pH drift with SWC's:

http://www.poolforum.com/pf2/showthread.php?t=1188

My pool is running closer to the typical SWC operation now that it is in full use. My ph does drift up now over 2-3 weeks (give or take) from 7.2 to 7.6.

When I do knock it back down, I'm having to use a full gallon of acid instead of the 30 oz. the bleach calc predicts, so my water appears to be pretty highly buffered.

Regardless, I absolutely love the SWC. I never dreamed pool maintenance could be this easy.