how vesting period affect pension expense?

my take on that is if you increase vesting period, the expense is reduced, b/c few people are eligible for the benefit. Any inputs?

I think the longer vesting period should decrease a vested benefit obligation, but not change ABO or PBO.

Wouldn’t the pbo decrease as well, because less people would be eligible for benefits?

No, because PBO is not based on how many are currently eligible, but how many will be eligible in the future. PBO even includes future salary increases, which neither ABO nor VBO do.