You only need to add the sugar not more yeast. I would think that adding vodka would change the taste. You should go ahead and buy a hydrometer so can get the end result you are looking for. You can add 3 lbs of sugar to increase the alcohol to around 10%.
What did you end up doing and how did it turn out? I know how to increase the alcohol but I don't know how to make it less sweet without messing up the cherry flavor.
My first wine
The first wine I made was a Riesling from a kit...I topped it off with waaayyy to much water and it is really bad. However, I recovered with another kit wine and 3 batches of skeeter pee that turned out fantastic!