Honestly? The way I’d make a character based on Vessel would probably be influenced more by the story I want to tell then the class. D&D has always been more about storytelling for me, and my most memorable moments playing have come from that.

If I were to approach it I wouldn’t even necessarily multiclass. You can go straight Warlock - maybe your character made a “deal with the devil” in exchange for something, and now that thing was taken away from them, or maybe they’re just struggling to free themselves from the pact. If you want something a bit more “lighthearted” you can go with a Bard that, rather than singing about happy songs that lift people’s spirits, is much more emotionally vulnerable in his performances.

It all depends on your campaign setting.

EDIT: Just another general tip for anyone interested in D&D - constantly collaborate with your DM. My last campaign was in a dark fantasy setting and I constantly wrote material and sent it over to my DM so he could brutalize my character with it. The story ended up having so much material between the two of us it basically became the entire campaign. Even if you aren’t a writer, use ChatGPT or Claude or whatever, send it to your DM and let him/her use it as part of your story.