Final Average Earnings: Can Post-Severance Comp Be Counted in Final Month of Employment?

[Übernerd]

Plan A is a traditional DB plan with a Final Average Earnings (FAE) formula. It defines FAE as the average compensation for the 60 months of Employee's last 120 months before retirement that produce the highest average. Plan A counts post-severance payouts of accumulated vacation pay in FAE. Typically, it's paid in the month after the Employee terminates. E.g., if Employee terminates on 1/15/15, the vacation payout will be made around 2/15/15.

Is there a legal impediment to counting that February payout as part of January's comp (the last month for which Employee received a regular paycheck)? This would be more favorable to Employee's FAE because it would spike January's comp, rather than marooning it in February. I can't find any authority one way or the other.

Thanks for any replies.