Are bullletproof vests only effective against small firearms or also against some higher caliber weapons?

Question

If so, to what extent do they provide assistance against those of higher caliber and if not, why can't vests be manufactured to respond to these weapons?

Answer 1

No, they are effective against higher caliber but more depends on the nature of the bullet than the caliber - a very dense very sharp bullet will penetrate where a large soft slug will not (in musket wars, heavy cloth coats served as "armor")
Body armor is available for high penetration high caliber rounds, but the trade-offs are that it gets heavy and stiff making maneuvering difficult and protecting the face is difficult. At some point the person can't move and a solid structure is required - an armored personnel carrier or personal shield.
And with really powerful rounds, a rigid armor stopping the bullet would still knock the person off their feet and perhaps leave them with concussion damage as their head snapped in reaction to the blow.
Besides cost, the problems of carrying the weight and the relative rareness of really dangerous rounds reduces the market.