I would be careful about that because Lexapro is supposedly able to make one drowsy. It doesn't make me drowsy but it might strengthen the effect of Valium. I couldn't sleep my first night on Lexapro either because of the stomach "nausea." I think the word should be feeling like you want to throw up rather than "nausea."
If you want a good answer on this, stop by and talk to your pharmacist. Doctors are hard to see but a pharmacist is easier to access. I would talk to my pharmacist or doctor before trying Valium and Lexapro. Does the doctor who prescriped Lexapro know that you are on Valium? My doctor ask me what else I was on before she prescriped Lexapro. I hope you sleep okey tonight. If not, I managed to sleep better after the first night. The stomach upset decreased enough for me to sleep with Lexapro in my blood stream.