"Si vous voulez" or "Si tu veux" can be used independently as a reply to some proposal. -On va manger? -Si tu veux. The meaning is close to "D'accord". It is an indication that you wanna make sure that both you and partner agree to do something (but it doesnt call for an additional "Oui, je veux"!). By extension, it can also be used for expressing or requesting understanding for/from your partner when the formulation of an idea or of an analogy is not totally adequate. For instance, u re talking about checks, and trying to explain to a friend who doesnt know: "Si tu veux, c'est une sorte de / c'est comme un billet de banque, (mais...)" If somebody says something to you that u dont find completely convincing, u can also comment: "Si tu veux...". In that case, it expresses ur doubts, but also that u agree, at least temporarily.

"si vous voulez" always means "If you want" but the meaning can be slightly different according the way you use it. For exemple in this sentence : Si vous voulez du thé demandez moi = If you want some tea ask me "Si" introduces subordinate clause which outlines the necessary condition to see the action become true. But there is an other way you can use "si vous voulez" It's when you want someone to do something but you ask him in a polite way. For exemple : Si vous voulez bien me suivre= veuillez me suivre=Please follow me

I don't understand your question very well. "Si vous voulez" means: " if you want" and can be used in any context. Can you clarify your question?