The ideal VPIP for .5/1 is probably between 17 and 19, but any more means you will get reamed by the rake. Given that, you aren't going to enough showdowns. When I played .5/1, my VPIP was 20, but I was going to 34% of showdowns, and winning just as many of them. This may be that you are running bad, but it is probably a result of folding too much.