The vaccine only triggers the body to fight the disease.
How many 'T' cells it triggers in your body is the only relationship. How well your body will fight the disease and prevent an infection is the work of the 'T' cells not the vaccine. Pfizer et all, say that even one dose will create enough 'T' cells to do that and prevent you from getting hospitalized ..ergo 100% effective.
How well the vaccine works at preventing you from getting the disease and spreading it (it's primary purpose) is called its 'efficacy' rate...somewhere between 63% and 93% pending on the manufacture and how many doses.
That is why even though you've been vaccinated...you can still get the disease BUT your 'T" cells from the first vaccination prevent you from getting really sick from it.
Unfortunatly the numbers indicate in 'some ' people who have been fully vaccinated, can still get Covid (Delta) and they still get sick and end up in the hospital...so far about 20%. or 1 in 4.
edit add: Today in Ontario..x61 in Hospital....(44 un-vaccinated, 10 one dose, 7 fully) so 28% have been vaccinated