I recently purchased a 12mm plenum spacer for an 02 vu commodore ute 5 speed. After fitting my ute started up very rough and after a few seconds it started to smooth out but idle like it was slightly cammed but wasn't idling rough just sound. Drove it and

Given what you're describing and the changes you've made, it could be normal. However to be certain, assuming that you've installed the plenum spacer as shown in the link below, then you may have a vacuum leak.

http://www.maceengineering.com.au/WebRoot/ecshared01/Shops/mace/510F/07A7/ACFB/1D04/4CC3/AC10/000D/B2A1/PLENUM_SPACER_FITTING_INSTRUCTIONSmkii.pdf

In order to be able to check for a vacuum leak, you can spray suspect areas with soapy water. For further details about this process, please refer to the youtube clip below:

https://www.youtube.com/watch?v=LcY_ORcjgfM

Please let us know how you go with this and we'll take it from there.