Mandarins soaking in Vodka
The fruit is probably not fermenting. That alcohol level will kill most bacteria and mold, and while it wouldn't be entirely out of reach of acetic acid bacteria, the floating is much more likely to simply be caused by the segments absorbing ethanol and becoming more buoyant.

I would expect the liquid to taste fine, though I think you could get better and more convenient results by just adding some fresh mandarin juice to vodka. When liquor is infused with citrus fruits, one generally uses the peel (or at least the zest) as well or instead, because the ethanol is effective at absorbing the aroma compounds in them. Limoncello, for instance, is made from lemon zest, and Campari is made from whole chinotto.

In fact, if I saw a kilo of mandarin segments soaking in 500 mL of vodka, I'd assume that the primary purpose was to infuse the segments for eating. (I've seen this preparation called "tipsy oranges".) Of course, you can also drink the liquid. :)