Dehumidification of a vented crawl space is similar to running an A/C in a screened porch. It MIGHT have an effect, but it is a very, very inefficient way to work.
First, get a really, really good poly all over the floor. My advice, do this (or get a handiman to do it) and then monitor the situation.
Sounds to me like it's a poor circulation issue, and additional vents, powered, may only make it worse as moving air around by design pulls additional air in from other vents. I don't know what region you're in, but especially this time of year, more air moving in from outside means MORE humidity.
If a good poly doesn't settle things down, then seal up the crawl vents and use a dehumidifier.
Pictures would help.