You'll have to clarify as to which stat you're having issues with. However in general I can say that starting statistics are randomized within a range, which means that you will sometimes have 11 leg strength and sometimes start with just 9. Arm strength is mostly only improved by wielding heavy things (such as two-handed swords, maces, and certain quarterstaves), while leg strength is only improved by carrying a lot of weight (you'll see the fastest improvement in this stat while stressed).
Vomiting can be useful for making metallic walls easier to dig through. For example, there is a certain 3x3 vault with no door made of meteoric steel. If you don't vomit on it and don't have, say, an adamantine pick on hand, you're not gonna be able to get through to the chest inside. However, with a few V's on the same wall and a bit of waiting, the wall will rust to the point where you can hack through it with a copper pick. Vomiting on an altar of Scabies also increases your favor with her (though why you'd want favor with her when there are far more useful gods/goddesses to talk to, I don't know). Other than that, vomiting also can be used to decrease your satiation level (which might be handy if you don't want to be bloated/overfed).
Infravision allows you to see warm-blooded invisible creatures, even if they don't possess a mind (which is a necessity for someone using ESP instead).