Vata's goal deep into injury time came after Edo Kayembe had equalised with a penalty for the hosts - although replays showed contact between Kwadwo Baah and Pompey's Freddie Potts occurred outside the box.

Zak Swanson's first goal of the season had given the visitors a deserved half-time lead.

The win moved Watford, who had Kwadwo Baah sent off after the final whistle, up to sixth in the Championship table, while Portsmouth are 21st.