If I ever have a medication question, I call the pharmacist. I would do that, if I were you.
I don't know of any problem with taking Zyrtec-D with Plavix. You can also read your medication leaflets with the medications to check or check with your pharmacist.
If you are being treated for high blood pressure or heart disease/vascular problem, I would NOT recommend taking the Zyrtec-D WITHOUT discussing this with your physician.