Thank you so much! I think I have it, although it seems very easy, which always seems suspicious to me in maths. Here goes:
Ʃ |<x,vj><y,vj>| ≤ √(Ʃ<x,vj>2) √(Ʃ<xy,vj>2)
Then by Bessel's Inequality
√Ʃ<x,vj>2√Ʃ<xy,vj>2 ≤ √||x||2 √||y||2
So Ʃ |<x,vj><y,vj>| ≤ ||x|| ||y|| as required!