Managed to figure it out. Apparently you can't have both the vehicles.meta of update.rpf and the vehicles.meta of RDE together at the same time. I guess the update.rpf vehicles.meta takes precedence over that of any DLCpacks.......removing the custom lines from the .meta from update.rpf and leaving the lines in RDE's vehicles.meta fixed the issue, a dumb and oblivious mistake by me.
What is kind of odd though is that I added the custom addon lines into RDE's .meta shortly after I found out it existed, I figured adding those lines would of made the vehicle spawn, but I guess having the vehicles.meta in update.rpf having the same custom addon lines just caused more confusion. Oh well, I learn something new every day.
Thanks for your help, it's very much appreciated!
EDIT: By any chance, would you know if the vehicles would of spawned naturally in traffic had of I not of fixed the vehicles.meta issue? As I've got the new addon cars spawning in the same places as the other Hwaycars.