And it's not just that the gene can spread. New broken versions of the gene will keep popping up everywhere all the time. Without a mechanism to sieve those out, they will eventually drown out the working versions by sheer numbers.

You could also say: the advantage is that your offspring doesn't have to die if the gene has mutated.

If something is enough of an advantage, it can be strongly retained as mutations which deactivate it don't survive well.

If something used to be an advantage but now isn't one, it gets really complicated.

If something is a disadvantage, it is almost always lost as mutations which deactivate it spread much much faster (just another type of advantage).

The neutral one, often called "relaxed selection" is when something WAS an advantage (like synthesizing vitamin C) but isn't any longer. What happens after is super complicated. Sometimes it is retained, sometimes it is seen in a stable percentage of the population, sometimes it disappears completely. We are like....really really bad at understanding and identifying that case, doubly so when the subject isn't something big and easily studied (like losing eyesight in lightless caves) vs something so seemingly small with a lot of reliant processes and interactions (the ability to synthesize vitamin C through the GULO enzyme). Some people think the useless genes will stick around until there is a strong selection against them, some think that mutations will slowly be eroded in the population until it becomes so horrible it can't reactivate. I dunno. The interaction and transformation of genes into and by pseudogenes is a leading edge, debated subject.

So realistically, not only have we not yet found a definitive clear advantage to not producing vitamin C, we also don't even really understand how to predict what happens to things that were an advantage but aren't any longer, and we keep finding that they actually have some selective reason behind being lost after all (blindness in a cave is now thought to be probably advantageously selected for).

There are really cool studies on bats (most lost the ability to synthesize vitamin C, but some still have it, some seem to be in the process of losing it, and some lost it REALLY recently, like in the last couple million years).

Other cool studies are with Mexican tetra, which have both blind and sighted version that can interact with each other (the cave systems connect with outside water systems).

What surprises me about that is that I read before that Vitamin C to most animals tastes really bad, but not to primates because we need it to survive. So if losing endogenous Vitamin C production was just a case of "it doesn't hurt", it would mean that primates consumed enough Vitamin C through their diet before losing the ability that it didn't matter, otherwise it would have been a disadvantage, right? But at that point they would have probably disliked the taste.