There is absolutely no reason not to add more Vodka to your tinctures, just make sure to shake them several times a day. I always end up having to add more to mine and they always turn out great.
I don't know how much you've made of each tincture but remember that you don't have to press the whole jar, just press the amount you think you'll use until the next full moon. That way the rest of the tincture will get that much stronger when you leave it tincturing. I've got many tinctures that I did in October of 2009 that I've only pressed what I need at the time so what's left in the jars is going on 1 year old YAY! Those are some very STRONG tinctures indeed!