Hi
We have an ancient North Face Frog (the tadpole's big brother) and it is a robust tent which is still going strong. I don't know the Vango but things to think about are;
Weight - with a 4 year old you are carrying everything, it looks like the Tadpole is about 2kg and the Vango 3kg.
Price - as soon as something is secondhand the price drops like a stone, make sure you are not subsidising your mates next hobby.
Commitment - have you and child done much walking and camping before? It is not for everyone. Could you get a couple of days free trial of the tadpole before you decide on anything?