If it's any help to you missperelman I had single vaccines for DS1 when he was 13 months.
This was because I was scared by rumours of autism, caused by that silly doctor. DS1 had rubella vac first and then measles vac 5 weeks later.
When it was time for his mumps vaccine, I was informed by the Children's Immunisation Centre that there wasn't any, due to non manufacture in favour of swine flu vaccine. They said they would be expecting a shipment the following year. That never manifested and so he has never had a single mumps vaccine.
When DS1 was 3yrs 4 months, I took him to have an MMR (pre-school booster) and he was fine. So, vaccine- wise, he's covered, but only has had one dose of the mumps.
DS2 has had mmr's both times. I didn't bother with single vaccines with him, as with hindsight, it was a waste of time and money.
Before anyone has a go at me, yes, my DS1 was not fully protected from mumps, but if the autism thing was a real possibility, I would never have forgiven myself. (I did more research into it and thus was happy for DS2 to have MMR both times)
I have spoken to a paediatrician about this, who said that two doses of the mumps vac is recommended, as one dose only causes immunity in about 70% of children. The second dose is to bring that % closer to 95% out of those 30% who didn't get immunity the first time around.
I am still hoping that a univalent mumps vac will become avail, but if not, then in the summer, DS1 will go for another MMR to cover him fully for mumps.
Again, the paediatrician said that I should not worry about him having a 3rd dose of measles and rubella, as they will do no harm.
The point of my story is that I really wouldn't go down the single vaccine route, because you CAN'T get hold of a mumps vaccine in this country. The Children's immunisation Centre still haven't got any. DS1 is now approaching 6 and I won't wait any longer.
If you think about it, thousands of kids have been mmr'd with no serious consequences that have been reported ( as far as I'm aware).
Don't give yourself the heartache of the single vaccine route because it will only cause more injections than necessary.
Of course, you must make your own decision for your child's welfare, but as someone who has done what you're wanting to do, it really is not worth it. Go for the MMR in my opinion.