Hmm. I think the answer is in the first part of your original post. Something about using too much epoxy, etc. Now that you've got that first one finished, you should be able to shave off several pounds through improved technique. How heavy was the first boat? Most stitch and glues can be built in the 40-48lb range by just following the designer's instructions carefully and using good layup technique. The benefits of vacuum bagging are generally advertised as a combination of weight savings, up to 30%, and strength, stiffer layup. If you're REALLY good, you might take off 10-12lbs. which might not be so noticeable in the water. Sure would make putting the yak on cartop easier.

I've used the shopvac bagging technique to build RC racing sailboat hulls out of carbon fiber and kevlar (50" hulls). It was quite a learning curve and the first couple of hulls were definitely not ready for prime time. So, evaluate the odds that you're going to get the results you want the very first time on the boat you're wife is going to paddle. All that said, it sure is a lot of fun to experiment with new approaches.