I can see the rationale in one sense in that Virgin has the brand and distribution strength in the UK to generate feeder traffic from the UK regions which they need to support long haul and by wet leasing they are not committing much in terms of capital risk.
However the revenue they will gain will be marginal whilst adding cost and complexity to their operation.
Seeking the IAG slot pairs for Moscow is entirely rational as the route wil yield good quality O&D traffic and feeder traffic. I would think Virgin would be better going for new medium/long haul routes that can provide both direct and connecting traffic for their existing network.