I would also reuse it. It's vomiting and milk as much as urine and faeces though. If you're happy the whole mattress has always had a waterproof protector then I would personally be perfectly happy to reuse it provided it wasn't sagging and still fit tight to the cot bars.
It does still fit really tightly to bars and when I was putting the fitted sheet on I was thinking it looked in really good condition and a shame to throw it out. There wasn't any milk spilt as ds was breastfed up to 6 months and then had dream feed until 1 & so didn't have a bottle in bed. Sure he must of been sick but was always washed down.
We have two washable waterproof mattress protectors which are like a fitted sheet and is a god send. You can just take it off with the fitted sheet on top, put a new set on and wash them. You just have to remember not to tumbledry the waterproof one.
If the mattress is odour free then I'd call it clean. The SIDS risk is from bacteria in the mattress; a soft sagging mattress, gaps between the mattress and the bars and signs of deterioration [springs poking through etc].
If there are any smells I'd give it a good scrub/steam clean and make a call then.
I agree with everything everyone has said but decided to buy a new cotbed mattress. Put simply we could afford it and I wouldn't have been able to forgive myself if something had happened to dd2. Even now I know it was a real heart over head decision but there goes. Also it did help that I knew I could pass the mattress on to a good cause. I was fortunate to be able to make that choice.