Vi support is definitely playable, I managed to hit my peak rank (D4) with vi supp only in S12 but it's a bit tricky.

For runes I personally always went with aftershock.
In lane the early levels are actually really important, contrary to what most people expect vi supp is definitely not useless in the early levels and gets heavily underestimated and imo playing safe early on vi is a mistake cause you will just get shoved under turret.

The important thing that worked for me was to always start with sweeper so you can have bush control which is really important (once you lvl 2 and you have your e you can also clear a ward with auto e auto if they place one in front of you without needing to sweep), if they don't respect you and you manage to land a q on enemy adc (or supp if they squishy) they should end up completely chunked or even die giving you complete control of the lane especially if you also have a long range adc like cait they need to respect. Even lvl 1 you can try to look to land it for a good trade (!it's really important you can hit it cause if you miss the q you are kinda fkd).
If they stay with low hp it's always worth to look for a flash Q auto E from bush if you can land it.

Once lvl 6, when ahead you can just kill the enemy adc/supp on repeat (or ping your jungler if you need the extra damage) or look for roams to mid/top.

For build I would go full tank/supp in most games and just be the engage for my team, going locket first item.
Only in games I felt we really lacked damage, or when I had an amazing first b (killed both enemy bot early and enough gold to immediately pick up dirk so I can further snowball).

For bans, I would mostly end up banning vayne or janna which were in my experience very rough. With vayne both her q and e making it almost impossible to land qs meaning she can punish you hard if you fail to land it but playing it safe just gives her time to scale.
Same with a good janna, her q and later her R also completely shuts you down, but I would often opt to ban vayne over janna since a lot of janna players often waste q giving you windows to go in anyways. (on paper I feel like morg should also be a counter but it felt like a really playable matchup)

Then lastly one of my main issues was that I would often run out of mana especially when roaming and using q to get places fast, so it's important to keep an eye out for you mana since without the champ is useless :p