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!
Hedda has known Eilert in the past as a libertine whose Saturday night visits to dens of iniquity he would describe to her during his Sunday afternoon visits to her home. She eagerly imbibed these accounts, which made debauchery seem a sort of perverse blessing, setting him free from the inhibitions restraining ordinary, respectable people and giving him the "courage" (in his own words) "to spit in the eye of the world." In his youth alcohol appeared to kindle his dazzling mind in the manner in which the Dionysus, god of the vine, was believed by the Greeks to inspire his followers with creativity and liberate them from mortal limitations through ecstasy. The vine leaves represent Eilert's allegiance to Dionysus; they are the crown of freedom which Hedda imagines alcohol to confer upon him.
This is not so much an "answer" as a comment on what's been written so far. One of the most interesting HEDDA GABLERs I have encountered was a filmed version made (I assume) by the BBC in which Ingrid Bergman is Hedda, Michael Redgrave is Tesman, Ralph Richardson as Brock and Trevor Howard as Lovberg. My comment is simply a matter of pointing out that Trevor Howard was an alcoholic - if it had ever been a secret it was definitely not a secret by the time this production aired.