What do you think Ursula would've done, if Eric and Vanessa (Ursula's Ego) got married?

well she would still turn back to being a mermaid and cry her eyes out until she heads to atlantis

Yeah, I think that Ursula would've killed Eric to scar Ariel permanently.

She would have continued with her plan to force Triton to give her his crown and trident, and maybe kill Ariel when she resisted. Either that, or Eric would rock her world so much that she'd forget all about that plan to take over the sea.

Either way, she'd keep Eric around as her extremely enjoyable plaything.