I'm not sure about the vodka addition as I 've never done it. Vodka is pretty neutral in taste though but if you use a good vodka, that's an expensive way to increase alcohol (I wouldn't add anything I would not be willing to drink on its own; cheap vodka has a bite to it).
I have made the mist type kits at 5.5 gallons instead of 6 (more for body than alcohol).Then I topped up with a commercial wine. That wasgood.
I have also added sugar to bump up the alcohol. You should allow a bit of aging time if youincrease the alcohol. I added 3 pounds of sugar to a rasp white zin at 6 gal volume to bring the s.g. up to 1.080 (and gave it 3 months aging)and it was good. I don't think it affected the flavor but someone else I know did the same to a different mist type kit and thought it wasn't as good as the same kit made at 5.5 gallons. So it's all personal taste.
I have even heard of people adding 5 pounds of sugar but that was usually to something like a blackberry merlot.I haven't tried it myself.Some say it's good that way and some say it getsa "hot" taste to it.