Actually, some relationships -are- like that, but they're FACKING BORING >:I
It's a shame we didn't get to look more into it, but hey, as you said yourself, it might be an idea for a little fanfiction of yours :P
And yeah, I completely agree. 'I love you' is supposed to be an incredibly powerful phrase, and shouldn't be squandered on, like... "I'm gonna go fish a bit" "Okay, I love you :D" It doesn't work!
If Vincent and Victoria ended up alone on the island, with a proper shelter and everything, Vincent could've been pulling aside scavenged items that he'd found along the shore and in secret begun putting together something for Victoria.
She might've found out about his hoarding and just accused him of taking everything for himself. The next evening, he takes her with him to a spot he found. Secluded, with a view out over the ocean, possibly on top of a cliff somewhere. He'd have used the stuff he scavenged to put together a proper, delicious meal for her and finally be able to tell her "Victoria, I'm... sorry I took all of the stuff. It was selfish, but I felt that I could never put into words how much you contribute, and I never feel I show you how much I appreciate it. So here's me trying to do it without those words. But now that we're standing here, it's... alot more clear to me how I feel, and what I should be saying. Victoria, I... I love you."
Annnnnnd scene. Not the most perfect moment, but it's alot better than just squandering the phrase!