Just to make sure that you get the things right.
Valya is not a "pet" version it is just a short version of the full name (Of course it can be pronounced with a sweet voice that can make it a "pet" version). Like Will for William or Bob for Robert.
While "Valentinka, Valen'ka, Valyusha, Valechka" are the "pet" versions (diminutive ones). Personally I would never use "Val'ka" (Валька), It doesn't sound nice to me.
In such a situation a "pet" version of the name would be certainly the best.
If your Dmitri had left the Russia when he was about 7 to 9 he hardly was accustomed to say any terms of endearment to a girl other then some versions of the girl's name.