Just to be clear - Virgin America and Virgin Atlantic are two completely different airlines.
Virgin America are the ones who currently serve PDX, and are losing a tonne of money. They're realistically likely to stop flying in the next few years unless they can turn their financials around quickly. They are a short/medium haul carrier, whose routes are entirely in North America. Virgin Group only own 25% of the airline, the maximum shareholding permitted by US law.
Virgin Atlantic are an airline based in London, who are now 49% owned by Delta. They're primarily a long haul airline, serving destinations to and from London. It would be nice if they served Portland, but it's unlikely to happen given slot constraints at Heathrow. They would almost certainly have to cancel another route to serve PDX.