What defensive items do I get as a squishy mid/adc if I want to stack armor?/

Randuins, since it gives armor as well as health, is okay on ADC's especially with the passive (good for dueling other adc's) and active (peeling/chasing especially if you have BORK to chain it).

Guardian Angel is typically best, since most ADC items don't give great defensive stats and GA gives mixed defenses as well as a revive.

On midlaners, zhonyas is your best bet. After that, you're best getting health since it's the universal defense (except against %max damage).

for AP champions pretty much for everyone if you really need to

i built ga before n theyir ad threats just shredded me anyway qq

Banner of Command and GA are the only viable things to build on a mid laner.

Really depends ons the individual champion are all good options for AP champs. And a lot of ADC have AP abbilities, some even good ones.

For Vayne some of these may be options