Vishnu as Ishta Deva and Bhairav Naam Jaap Mantra (Help Needed)

Inspired by Rajarshi Nandy Ji's words, I started doing Naam Jaap of Lord Bhairava (om bhairavya namah) on the 22nd of December, last year. I do about 5 malas, everyday and have already started to feel some changes.

I recently found out that my Ishta dev is Lord Vishnu.

How do I go about doing Naam Jaap? Should I switch to om namah bhagwate vasudevaya or can I do both?

Hoping someone can help me out here.