I took my prescribed 30mg of Vyvanse (lisdexamfetamine) approximately 7.5 hours ago, is it safe to take a Mucus Relief DM (dextrometorphan HBR 20 mg, Guaifenesin 400mg/ Expectorant) right now?

Yes. Assuming you take no other meds, combining Vyvanse (lisdexamfetamine) with Mucus Relief DM shows no significant drug interactions. Hope you feel better.
I would think so. Derek, I do not believe that you should have any trouble. The two should not interact with each other and I am not aware of any concerns. I am glad that you are cautious with possible interactions. Wishing you the best. .