Vassallo "will champion key relationships with digital media owners from a trading perspective and will ensure best practice for all client work on display".
He will work on client business for AXA, Nationwide, Hyundai Kia, TUI, BBC, Camelot and First Choice and will report to head of trading, Alan Brydon.
Vassallo joins from Diffiniti, where he was head of digital trading and headed the agency's Sky account.
He said: "I'm really looking forward to working on ways to make our clients money work harder and working with Media Contacts on their plans in the demand side space."
Media Contacts managing director Paul Frampton said: "Paul's appointment comes at an exciting time for us, and Media Contacts are continuing to invest in building resource around new and emerging channels. In particular, we will shortly make a big statement about our demand side platform."
Last week, Aegis Media scrapped its Diffiniti digital agency brand in the UK, as part of a global rollout of its US-based iProspect search specialist brand.