I'm putting a fangame together and I wanna ask for Smogon's advice on building fakemons that'll be fun to play with.
What forum would I ask a question such as, for example, "What's the best possible typing for an assault vest user"?
There isn't really a forum dedicated to this type of conversation since Smogon doesn't endorse fangame discussion (the closest place is Orange Islands which talks about in-game, but talking about fangames is fairly restricted there). The precedent of TPCi sending C&Ds to forums in the past means it's better to be on the safe side, and TPCi is vocal about how much it hates fangames.
However, with good knowledge of Pokémon I think you can work most of these types of questions out yourself. Perhaps read some of the threads in CAP (Create-A-Pokémon) on this site to see how the community works out how to optimise Pokémon designs for battle use, since they're pretty much pros at it over there. You could also try bringing it up in Smogon discord channels since conversation topics are lax there, but I'm unsure which one it would best fit into.
For your particular query, I'd argue the best typing for an Assault Vest user is one with good physical resistances that best needs to supplement its natural physical bulk with stronger special bulk. Then, look at the most common types used by physical attackers (Ground, Fighting, Normal, Rock, Dark, Dragon & Bug seem to be the standouts). The best type to counter this setup seems to me to be Ghost/Fairy, which only leaves a neutrality to Ground-, Dark- and Rock-Type and no weaknesses to anything. You could even include Levitate to alleviate its Ground-Type neutrality, which would fit flavour-wise since it's a Ghost-Type. Another option is Ghost/Steel, but that has a weakness to Ground- and Dark-Type in exchange for being immune to or resisting the other 5.
Another route you could take it is to have a heavily specialised special sponge. That means addressing the types most commonly used by special attackers (Fire, Water, Grass, Electric, Psychic, Ice, Ghost, Fairy I'd say are all common enough to be worth addressing. Maybe Flying too.) This is a more difficult type combination to come up with since these types don't share many resistances, but Grass/Steel is perhaps the best. This resists Water-, Grass-, Electric-, Psychic-, and Fairy-Type, while remaining neutral to Ghost- and Ice-Type but having a quad weakness to Fire-Type. Water/Steel is another option, exchanging its 4x Fire-Type weakness for 2x Electric-type weakness, which I think is less favourable because being weak to Volt Switch will hurt it in its role of eating special hits.
There are a few directions you can take Pokémon design and there usually isn't a "best" type to make a certain Pokémon. Use a bit of reasoning like I've done here, find some good options, and then just pick your favourite type that you want to design around. This reasoning is also over-generalised and it may be easier and more lucrative to design around specific Pokémon you want your OCs to beat, since otherwise I wager most of your Pokémon would end up sharing a type since some are simply better than others when speaking in grand terms. Good luck with making the game.