Made blackberry jam yesterday, and have 6 cups of pulpy juice left to do something else with. Might just make a syrup with it, but I'm interested in making a blackberry cordial

Online I find many recipes start with soaking berries in vodka, but that's out since I already have the juice. Some use brandy, some use vodka.

Can I make syrup first and then just add vodka to it when I want to make a cordial - in other words just use it as a flavoring for a drink?

Anybody had any experience with this, or something similar? Would appreciate any ideas/advice you can give me, including the amount of sugar to use.