Not done a vodka infusion myself, but I've made a few fruit infusions. What I've done is let the beer ferment out for about a week, then freeze the fruit overnight and add directly into the FV, or the Tesco water bottle secondary if you want to try a smaller batch, and leave it for another week. Make sure you fit an airlock though as you'll get some secondary fermentation from the sugars in the fruit. Or you can add the fruit earlier in fermentation, about day4 and just let it ferment out together with the fruit. I've done both with no notable difference.
The reason for freezing the fruit is that it ruptures the cell walls of the fruit, allowing more flavour to be imparted into the beer. It also helps with sanitation, it won't kill any bacteria, but it will put it to sleep. But if you add it once fermentation has started (i.e. about day 4) or after fermentation has completed, the beer will be alcoholic enough to protect itself from bacteria.
Although not having done it myself, with the vodka you could make up a liqueur by steeping pineapple in vodka for some time (maybe 3 weeks) to infuse the flavour and then add the liquid straight into the FV or bottling bucket right before bottling. You could also add the alcohol infused fruit to the FV too and let it steep for a few more days, or just add the liqueur and use the fruit to make a boozy dessert.
I hope this helps