Well a few points to consider. What exactly you feel as vibration may actually be noise as well. How "sensitive" you are to vibration, there are a lot of folks who seem overwhelmed and ironically surprised by the sensory inputs driving a small lightweight sports car can cause. That is to say everyone has a different idea fo what "a lot" of vibration is.
At that speed, hopefully on the highway, and assuming top is up you are dealing with:
Buzzing the engine in the 4K range, noisy but not a lot of vibration typically.
What condition the road under you is in, asphalt or concrete, left lane or right lane (trucks tend to make this worse)
Are your tires and wheels still in balance? I pay for a higher level of balance on mine 2016. The wheels bend easily if you have found any potholes. Most likely source of vibration.
Assuming a soft top you will get some vibration noise if the top is up. Top down less of that replaced by wind buffeting and tire noise.
Del Val Miata Club