You won't get an HCl by disolving it in vinegar. You will get an acetate salt. First try to dissolve what you have in a bit of water. If it starts to dissolve its not freebase. It's more likely that your vendor is talking out if his arse and doesn't know the meaning of the word freebase as I can't see any reason why anyone would want to manufacture an NBOMe that wasn't a salt.
If it really is a freebase, it won't dissolve, not even a tiny bit. It will just float around in top. Add little drops of dilute hydrochloric acid until its all dissolved and then evaporate the water off.