Well i just got to T3 access on all 3 venders,, heres what u need,
for T1 - T2 From Romaine u need to buy Enough items from Her that the Value of Each Items will Reach or exceed 7k Value,, so example Hypos are worth like 200 each so u would needaround 350 hypos,, thats alot,, so just look at the value of the items can buy the Phaser Arrays and get T2 access in like 6 - 8 buys
For T2 - T3
You have to Get a Value rating with each vender
For T2 Engineering Vender u need to buy enough items to Reach Around 300k,, buy engines is the easy way
to hit T3 on that Vender
For T2 Science Vender u need to buy 300k worth of items there also to hit T3,, Deflectors easy way
For T3 Tactical Vender u need to buy around 100k worth of items,, reason why 100k cause no item is worth more than 8500,, i bought Plasma Torpedos cause there were alot on the Exchange
and thats what it took for me to Hit T3 access on all 3 venders,, i hope this Helps