Ashe haves low early game as vayne ... you can choose maxin E if you have a passive support and try to farm all time with an avarice blade early (+7g every minionkill) or maxing W for ultimate poke if you have more agressives support's like Leona next to you :)
The most important thing you have to know about playing Ashe is good timing with her arrow ® . A good arrow can change everything in teamfight !
About building ashe : you can start with a dorans blade if you have a healer support next to you or if you have lifesteal quintigences (4%,6%) or boots + 3 healing of long sword +2 healing if you need the damage output .. first b i reccoment building avarice if you dont have money for stronger items .. if you can handle the lane stay till you farm money for BF Sword ! go b buy bf and some healings maybe some manas if you need them ! first take your Infinity Edge or you BT if you need more sustained damage ... Then go strait for Phandom Dancer except the enemy have more than 100 armor ... in that case you should build Last Whisper :) then take your phandom if you took the LW or the oppositt :D
final defensive item can be Wormogs , Randuins Omen , GA or Bunshees Vail ^^ [if the enemy has lots of cc buy QuickSilvers]
Now for vayne ^^
vayne is one of the strongest adc's in the game right now next to varus , graves , trist , ez
her third hit true damage % is great for killing high armor enemies and scalling EXTREMLY GOOD with atk speed ...
starting items can be same as Ashe's
Well as vayne with her Q you can escape anything in good timing ! a skilled vayne can change all the game with her survability and possitioning in the team-fight (thanx to her R+Q Stealth)
You can max W if you scale first atk speed or Q for more damage EARLY on game (note than vayne needs the early damage)
also this new OP item Blade of the Ruined King is Freaking OP in Vayne :P
so .. with vayne try to buy at first Back Cutlass from BOTRK and some atk speed maybe the entire item if you have gold ^^ (BOTRK has a great active too scaling perfect to adc's)
then go strait for Inifinity Edge , then LW , phandom and then a defensive item like GA , Randuins , Wormogs , Bunshees ^^
(NOTE. if you buy BOTRK its better not to buy + BT these two items do NOT scale well)
Well, my main role is adc's if anybody needs something else messege me ^^