"Solicitor tells me that he can't close the sale until vendor agrees to pay judgement."
A substantial minority of solicitors would take this attitude, but I do not agree with it. If it is a judgment mortgage (i.e. a judgment which has been specifically registered against the property) then it would indeed be an insuperable obstacle, but a mere judgment is not a bar to completing the purchase. Certainly it complicates matters a little: If it is merely a judgment, as opposed to a judgment mortgage, ask your solicitor to get an opinion from counsel on the steps open to you to complete the purchase. Expect an extra €600 or so in transaction costs.
Should your solicitor have seen this coming? No - closing searches are the norm; earlier judgment searches are not the norm.