Leather is just a material used because it's plentiful and cheap regardless of its origins, and if it were not so plentiful and cheap, it would rarely if ever be used.
Honestly, it's still a gray area, but I personally look at it like this: No one is going to slaughter a cow just for leather. Yes, buying leather supports the industries involved with factory farms and all that, but if no one ever ate beef, no one would make leather materials, at least not unless the price was jacked waaaaay up to make it economically prudent (and I suppose since there are people dumb enough to spend $2k on a pair of shoes or $4k on a jacket, it might have a tiny market).
Thus, even though I always prefer to buy an alternative if I can afford it, if I buy a leather belt from a thrift store or some discounter I'm not gonna feel bad about it (and I know there are vegan alternatives but to this day I've never found a single one that wasn't obscenely expensive, fell apart almost right away, or both - suggestions would be welcome for non-crappy products though!), nor am I gonna cry about getting leather workboots (same thing as the belts for me; ordered twice from vegetarianshoes.com and both were uncomfortable and honestly terribly made shoes - suggestions also welcome!).
Same goes for food additives, really. Geletain has a few alternatives that work perfectly, but they're not as cheap because the huge, swollen, federally subsidized beef industry makes so damn much that it's far more economically sensible to use it. This in mind, I'm not concerned about the implications of buying a tin of mints unless I buy a purposely veg*n version for usually two or three times the price, which, when I actually have the income, I do.
Same with flavorings an additives: If no one eats chicken, no one uses it in soup bases. In fact, what goes into a genuine article 'chicken soup base' is not 'edible' chicken so much as bits of the chicken would normally be thrown away, like bones, scrap and gristle. Sure, it's actually chicken, but unless nothing on the menu is made with meat, you are in no way supporting the meat industry by eating a soup made with 'meat stocks'. If you are avoiding such things altogether (since it ain't a bad health plan), you'd best just go to veg*n-only restaurants, but those are hard to find even in SoCal.
Personally, though, I don't like to drive myself batty by reading every label, I just try to buy and support 100% veg*n food and products when I can find/afford it. Otherwise it's not a big deal to me whether I've bought a tertiary animal by-product or not.
Besides, if you pay taxes in the US, you're already supporting all animal industries. Very, very sad, but true, so make damn sure to vote, people!
"I've seen a monkey evolve into a man,
I've seen a man devolve into a monkey,
I've seen a junkie redeem himself with help, and
I've seen a wealthy man melt into the snow and blow his credit on a decongestant."
- Sage Francis