Vacation allocation across multiple projects

For multiple project allocations in the paycheque module -- when I manually input the "Vacation Earned" amount, it gets split across all the projects (seemingly arbitrarily) --  is there a way to make sure that 100% of the "Vacation Earned" is allocated to just one project? Without adding or changing income types? We're using the max. 20 types and too many other employees will be affected if I turn off "Calc. vacation" for the necessary incomes (although, I turned it off temporarily as an experiment and got the same result). Thanks!