One more A/C problem to address

Sometimes when im stepping on the gas the cold air goes from blasting to basically a trickle between the floor and top vents (even though i have it set on just the top vents). Maybe this is normal since this car uses vacuum for a lot of stuff but I'm guessing this is a vacuum issue. I'm assuming that it just defaults to send air through the top and lower vents when theres not enough vacuum. So basically what i'm asking, is this normal or should i be checking my vacuum lines? (And would it most likely be where they connect to the climate control deck or is there any where else i should check.)

Oh and one more IMPORTANT thing. As you all may have heard my AC hasnt been in working order in about 8 years. And so, should i have to worry about siezing my pump or evaporator or whatever it may be? The AC recharge stuff i used said it contained oil but should i be adding any more? If so then what kind should i get?