Whoa, that’s a specific pickle! Been there with a few grimdark romantasy reads. Escape from Hell is less about brute strength and more about metaphysical loopholes, I think. There’s usually a pact or a bargain involved—trading something of immense personal value, like a memory of your first meeting or his ability to walk in sunlight, to a higher demonic power. The bureaucratic nature of infernal realms gets played up in stuff like 'The Demon’s Bargain' series; he might have to win a trial by ordeal or find a true name.
If your vampire is old-school, maybe he has an ancient artifact squirreled away, a saint’s relic or a celestial key, that can force a temporary gate. But the emotional cost is the real kicker—the escape often requires him to fully embrace his monstrous nature, which then strains the marriage. It’s a classic dark fantasy conflict: gaining freedom but risking the bond that makes him want it. Honestly, the messier the solution, the better the angst payoff later.