Port

IMHO you should use vodka and not brandy. Brandy as bought in your local Booze shop will already have a distinct flavour of its own and to add this to your "port" will alter its taste too. Vodka is without flavour and will not do this. I would also suggest that you use a vodka with as high a % abv as you can find an preferably not less than 40%abv. You don't mention quanties of port you have made or its current %abv so look up the use of the "Pearson Square" Also be prepared to bulk age it for a couple of years after fortifying it! The addition of neat alcohol makes the wine very harsh and it needs time to adjust & mellow out. I am currently drinking my 1996 Fortified elderberry ( 21% abv) which improves year by year so far. (btw I am only allowed a couple of glasses each week these days!)

I would also add that if you can get higher proof alcohol (like Everclear -