How difficult will Albanian be to learn if I already speak Modern Greek?

Yes, the vocabulary is completely different—except for the large number of Greek loanwords in Albanian, which is substantial, and the rather smaller number of Albanian loanwords in Greek.

OTOH: Balkan sprachbund. The syntax and inflections are remarkably similar: you can often translate Albanian into Greek and vice versa, word for word.

I’m reminded of what renowned linguist BRIAN D. JOSEPH once told me, when he was encouraging me to learn Macedonian. “It’ll be no trouble at all. You just swap the words.”