As far as I'm aware, there are 14 ex-BMI frequencies on London-Moscow to divvied up. Making an educated guess, it wouldn't take too much to work out that the CAA are quite likely to give 7 weekly frequencies each to Virgin and easyJet (actually not a bad result for BA as this would mean only having to give up 7 instead of 14 weekly LHR slot pairs).
That scenario won't happen. I'm pretty sure - though can't find a recent document to confirm - that the UK-Russia bilateral restricts the number of UK airlines allowed to links the countries to two. Since one is BA, then it would seem both EZY and VS couldn't win 7 weekly flights each. So it will almost certainly be EZY or VS (the BA bid for LGW-SVO stands no chance)
However regardless of any restrictions on the number of airlines, the amount of traffic rights VS wins does not impact on the number of slots to be divested at LHR. There are 12 slots up for sale and theoretically they could all be used to Aberdeen. They aren't tied exclusively to Moscow. There are just 5 for which Moscow is an 'eligible destination' along with ABZ, EDI, Cairo and Riyadh (and Nice?). Chances are if one or two don't get used for Moscow, they will simply get used to another eligible destination
The slots and traffic rights are totally different processes. The slot divestiture is being overseen by the EU. The traffic rights by the CAA. They could produce incompatable outcomes. VS could win the slots but not have the traffic rights. They could win the traffic rights but not win the slots and therefore need to source slots from elsewhere