If I take Vyvanse (lisdexamfetamine) will I get a rash?

Very rare. The stimulant class of medication (such as Vyvanse (lisdexamfetamine) or adderall) are rarely associated with skin eruptions, allergic or otherwise. If you have a rash and are taking vyvanse, (lisdexamfetamine) most likely it isn't caused by the medication. See your prescribing doctor if the rash looks like this one, or continues.
Possibly. Skin rashes are a very common adverse reaction, but the risk of it is no reason not to try a medication your doctor thinks can help you.
Very uncommon. Rash is uncommon. If you have taken an amphetamine before and had a rash, you might have one on Vyvanse, (lisdexamfetamine) but otherwise not much of a worry.
Unusal. Getting a rash with Vyvanse (lisdexamfetamine) is not a common side effect. You should have your doctor evaluate this rash .