"Vanek \v(a)-nek\ as a boy's name is a variant of John (Hebrew), and the meaning of Vanek is "God is gracious".

Given that "Vanek" is a variant of "John" - which is one of the most ubiquitous names for men in English and often used as a generic name for male strangers or randoms (à la John Doe)....my guess would be that this is just the speakers way of saying he met two randoms on the street and asked them the way to the partisan hospital.

He didn't exactly stop to get their names so just designated them "Vaneks".

*It may be Russian slang but I couldn't find any such information which would back this up, even after searching using various Cyrillic spellings:
ванек ваник ванук ванэк ванык.