Girlfriends biological clock ticking, wants child before marriage?

I'm dating a woman with one child (from a previous marriage) that wants another child, she's 41 years old. I'm 43 years old with 2 children of my own. I plan to marry her, but she wants to have another child soon (ideally before she's 42) and is pressing me to get pregnant even before we get married at the end of the year. I'm not 100% sure I want another child and if we had one and our relationship didn't work out than I'd be stuck paying child support. I'm thinking about getting a Vasectomy (realizing it could end our relationship). I really love her and she knows my concerns about having a child, but feel her biological clock is pressing her to try to get pregnant. She is willing to help me raise my 2 children and so I feel maybe I'm being selfish by expecting that, but not giving her the child she wants?