It is PUL AND bamboo vinegar is not good for, although as PP said you could probably get away with once or twice, just not regular. You could soak any non bamboo insert in a white vinegar and water solution overnight and then rinse them. For any PUL try washing them with a dishwasher tablet and then rinsing well, worked for me.
If bub is teething this could be the source of the smell. While mine were teething I pre-washed (or rinsed if you don't have cold pre-wash) and this stopped the urine setting in the nappy and got rid of the smells.
ETA: duh sorry, just re-read and you already pre-wash, strip wash it is then