I would be interested in an answer as well! (How to sideload the roon 1.8 APK on VITA?)
I registered my VITA to Roon v1.6. (With VITA on firmware 1.5 and 1.6), which worked really well - as supposed for a roon ready device. See my pics of the beautiful playscreens here. Since Roon 1.8, VITA is still hooked up to the Roon Core on my Nucleus, but marked „not certified“
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It should remain there over future updates.
Roon advised to not delete it in the settings; because I would not be able to register ist again … brave new world …