This one used to be a massive problem in Ibsen criticism and everyone went a bit crazy over it. It has since, you'll been glad to hear, been largely settled.
Vine leaves in the hair is traditionally a symbol of being a follower of Bacchus. Bacchus (often called Dionysus in Greek literature) is the god of wine, merriment, drama, and a sort of crazy, dancing, merry-making intoxication. He's all about seeking pleasure.
Lovborg, of course, is an alcoholic, and Hedda is referring directly to his alcoholism. When he returns "with vine-leaves in his hair", he will be god-like, elated, happy - or simply drunk. It's not quite clear whether she means this positively (i.e. visualising him like a god), or simply is saying that he'll be drunk.
An essay by Professor Dietrichson argues that among the young artists whose society Ibsen frequented during his first years in Rome, it was customary, for the party-goers to wear vine leaves. So perhaps it's got something to do with Lovborg being an artist too.
But it's about - I think - Lovborg's ongoing intoxication. And the ability it gives Hedda to manipulate him. And she sees him as part glorious, but also as part to-be-manipulated.
Hope it helps!