This Vi support is actually very strong. At all stages of the game.
It is also extremely fun and satisfying.
However I would avoid pairing it with weak early ADC. It is fine but a bit frustrating and counterproductive. You want a strong early and fight constantly.

About the runes. Glacial augment procs on q. It makes your q engage even stronger. Note that it reduces damage to ally by 15%. Meanwhile your w reduce armor by 20%. And bloodsong increases damage by 8%. Your ADC will thank you.

Glacial augment slow is increased further by 4.5% by revitalize. Note revitalize is bugged on self shield and increase them by 16%, not 5%. So your passive becomez 14% max health. It also increases shield bash damage.

Approach velocity gives 15% MS on glacial augment slow.

Biscuits gives you 90 max HP and make you even more of a lane bully.

Slightly magical boots... Just to get your item faster (no need to spend gold on boots early) and the +10MS is great. 420 gold value total (300 boots + 10MS).

Shield bash gives great damage with max health and shield. Your bread and butter for poke in lane. Synergize with revitalize.

Now the items... Heartsteel rush is great - bigger shield, good chunk of damage, tankiness, scaling... Randuin and kaenic are just the best (tankiest) armor and mr items, respectively, with amazing passives and active.. Unending despair stacks well with hp.

In lane you want to go in level 1. Hide in bush, engage with q, ignite, aa... They will either burn a flash or die. You might lose 50 hp but that's about it. Take a potion. Wait for your shield cooldown. Repeat... Deny as much cs gold and exp as possible, punish if they approach. With an early adc you will dominate laning phase. At level 6 you can dive them easy. If you shove or your adc recall roam to gank mid kr help your jungler.

Mid and late game you're a tank with cc... You can lock in enemy carry, soak damage, cc... And you bring decent damage for your team with heartsteel chunks, base damage, 20% armor reduction, expose weakness, etc. And reduce 15% enemy damage on your glacial augment ice. Vi mid and late reliable cc is why she is strong in competitive games.