You should have to distinguish between singular and plural "you."
"We love you (pl.)" or "We love you all" should be accepted.
Well since they're using "vos," you know they mean the plural and in english (regrettably) "you" can be singular or plural. Regardless, there are time when they should distinguish the quanity of the noun but i don't see why that's necessary in this example.
People familiar with Latin will know know they mean the plural. But because that distinction is not clear in English, learners can become confused and lose the distinction in Latin (a very fundamental one) because it doesn't exist in English.
Most text books I've used and taught from demand some distinction in translation, and I think that's the right way to go.
I agree it's not necessary. However, if they don't want us to translate it as "We love you (pl.)" they shouldn't even give us the option to select "pl." as a word in the word-picking version of this question. That's just unfair and misleading. I only added "pl." to my translation because they made it an option.