They stopped opening the legacy vault for quite some time now.
It got mentioned in this Ask Riot.
People didn't buy enough skins when they opened the legacy vault (which makes sense, how many people are going to buy an old skin and pay legendary tier RP if these skins don't fit the criteria anymore and are more like 975 RP skins?), so it doesn't make sense to keep doing these.
The only ways to get non holiday related legacy skins are:
- Hextech crafting (very luck dependant, wouldn't bet on it)
- Your Shop (doesn't apply here, since legendary skins are not included in Your Shop)
- champion specific events
So, as long as Tristana doesn't get a major update and they release all her skins again to celebrate it, the only chance for him is Hextech Crafting. Sucks, but that's how it is.