My advice would be to put your energy and concentration in the real, here, and now. Stay plugged in with your family as it is and choose joy in it. If a time comes when you both desire and are able to handle another child, then vasectomies are reversible with much less health risk than a mother getting hers reversed, or as others have mentioned, there are many children out there who really need a loving family to adopt them.
There is nothing selfish about considering finances and emotional strength or availability when thinking about adding to your family. In fact, I think it's the worst kind of selfishness that would say "I want a baby" and throw the other real concerns out the window without getting them in order first! It's one thing to find out your pregnant, realize it's a blessing, and make changes in your lifestyle to prepare for the baby.....but just saying "we'll figure it out" and trying to conceive first is just selfish and foolhardy, in my opinion. We were on birth control when we got pregnant the first time. It wasn't what we thought was the perfect time for us (debt, a 1 bedroom apartment, newly married, etc) but we were thrilled with our unexpected blessing and made the changes necessary, and went into overdrive to get it all in order for when our son was born. I think God knew what He was doing though----I can't imagine our marriage or family any other way.
That said: we PERSONALLY know, firsthand, of TWO couples who conceived even though they'd had vasectomies. I feel that if it's really meant to be, if you're really meant to have another baby, a little procedure like that can't stop God's plans. (Both families were surprised but thrilled). My husband had a V right after our youngest son came home, safe and sound, from the hospital. We made that decision mostly because of my health and safety: another baby...I might think of one occasionally, but am not going to risk my 2 wonderful boys growing up without a mom because I have second thoughts on leaving the "having babies" stage of my life. Money is also a large factor. We want to have a nice life, the boys we have going to college and debt free, some money set aside for our retirement and to enjoy the grandchildren....so our age and money are valid, not selfish, reasons.
I'm sorry you're having a hard time right now. It's probably pretty normal to have feelings of regret (I've questioned it a couple times in the last few years, but did not allow my mind to dwell on it or build it up, because my reasons are real and valid, and we did not make the decision lightly). My husband always imagined 3 children, we have 2, and I worried that he would feel "shorted". He thought about it a bit and said "Seriously, your health is so important to EVERYONE in the family. I have a full life, a full family, lots of love, and that's all I want. Besides, if we had another baby I'd NEVER get to retire". I hope you find peace in whatever decision you make.