Subsitute Yarn

I have a pattern for a basic waistcoat which is worked in garter stitch throughout with DK. Can I use Aran instead? Would i need to go down a size in the pattern due to the slightly thicker wool?

Probably. The way to tell is co about 20 sts and knit a couple inches with the Aran to see how many sts/inch you get. Then multiply that times the measurement you want and see if the st number is close to the next size smaller. Adjust the length to the correct size.