"je veux vous être" is not a complete sentence, it requires an adjective:
- je veux vous être agréable = I want to please you / I want to be nice to you
Typically in a store, you can hear:
- Puis-je vous être utile ? = Can I help you? (lit. can I be useful to you?)
"je vous veux" = I want you
"je veux être médecin" = I want to be a doctor
"je veux que tu sois mon ami" = I want you to be my friend
Note that "vouloir" behaves as a modal verb: it can be followed by an infinitive without a preposition.
Would you mind expanding on this a bit more? Why is "je veux vous être" an incomplete sentence even though "je veux vous aimer" is a complete sentence?
In general and for future reference, how would I know if "[pronoun] [verb] [direct pronoun] [verb]" is a complete sentence or not?
Direct object pronouns are placed before the verb.
State verbs (être, paraître, devenir, sembler, demeurer, rester) do not have a direct object, since they define or describe the subject and do not perform any action on anyone or anything.
"Je veux être vous" therefore keeps the "attribute" (this is the function of "vous") after the verb.
"Je veux vous être utile" has "vous" as an indirect object pronoun, standing for "à vous" and "à" belongs to "utile" - Je veux être utile à vous.
I'm curious if both are valid and duolingo doesn't accept one of them?
Translating with the word order intact would give "I want you to be", though if this were our intended meaning I'd guess we would need the adverbial pronoun "en"? E.g. "Je veux vous en être". Or, since in "I want you to be", "to be" is used almost as a noun, maybe something like "Je veux que vous être" would be needed there. Possibly relevant : http://french.about.com/library/weekly/aa032301.htm.