In Phrygian modal music, the II-i cadence is pretty common. Does the II chord here count as a tritone substitute chord that is substituting for vo?
No. Simply you are using the flavor of the mode a mode rather than substituting a chord a tritone. The flavor of Phygian comes out in the lower second rather than it being a chromatic substitution.
Let's talk about the simplest form of a tritone substitution. We have the progression Dm7 - G7 - CMaj7 (ii7 - V7 - IM7). We could easily change the G7 to a D♭7 yielding the progression Dm7 - D♭7 - CMaj7 (ii7 - tt - IM7). As you can see, this is very different than Phrygian progressions like i - II - III - II - i.
I will also point out a Neapolitan chord is another chord built on the lowered second and it would also not be considered a tritone substitution as it is functioning as a predominant.
The tritone substitution uses the fact that the 3 and the 7 in a chord (usually a V chord) are exactly the same sounding notes (not always written the same though!). Thus, V7>I in key C is G7>C. Notes involved in G7 are G B D F. Take the B and F, 3 and 7. Swap them over, so they become 3 and 7 of another dom7 chord. That makes it D♭7 - its 3 and 7 being F and C♭. C♭ makes the same sound (in 12tet) as B. So now we have the tts of G7 - D♭7.
Your idea of Phrygian II>i doesn't use the same formula,, even though the root note of II is a semitone above the root of i. That's not enough. Think about it - the other two (or possibly 3?) notes in tts aren't even diatonic! Phrygian II actually contains the same notes as the rest of that Phrygian mode.
Also, I've never used tts to arrive at a minor harmony. That's not saying it's not possible, just that I've never seen or played it. Perhaps someone can enlighten me there? Actually, that's not true. I've played B♭7 to get back to Am, so it must work! But that's key Am, not mode A Phrygian...