I just had a look at it, it's only rated to -3 so it's really pushing being called a 3/4 season bag, then again season ratings have always been a bit ropey and open to interpretation.
Given the choice and similar price I'd go with the Alpkit Pipedream 400 instead, it's lighter 750g vs 900g, equally as warm, bigger (unpacked) and compresses to about the same size due to using better down; 750fp compared to 600fp in the Vango.