Berserker's Greaves VS Ionian Boots of Lucidity
If you are wondering when to get AS Boots and when to get CDR boots,It is very simple

When you get Youmuu's Ghostblade get Ionian Boots of Lucidity

When you get Statikk Shiv or Phantom Dancer get Berserker's Greaves

If you are wondering when to get ghost blade and when to get shiv of phantom dancer it will be explained later

Bloodthirster VS Blade of the Ruined King
Now this is a delicate topic most people have their own theories on it,I personally would suggest that you go BT unless you are fed or your team is ahead cause imho BT just offers much more to varus than BOTRK to varus cause varus befits from sneeking spells and not in an all out right clicking contest.But then again if u feel that u need BOTRK to kite and stuff then buy it

Statikk Shiv or Phantom Dancer VS Youmuu's Ghostblade
The choice here basically comes down to your game play

If you are good at kiting and at skillshots go for the Ghostblade and CDR boots build cause it offers a lot of CDR on your Piercing Arrow and also offers some kiting potential in fights with the ghostblade active

If you just wanna kill em all with the right click the go for Shiv or PD and AS boots build,this is much more efficient when you have a team that can peel for you and you can just hammer away on the enemy team

Last Whisper And Infinity Edge
These dont require any explanation as they are built on almost any adc
Some tend to skip last whisper and never build it but i dont recommend doing that like the only instance where you can is when the enemy team has 5 squishies(by squishies i mean when there are no armor built on the enemy team at 40 min mark) and that too if u have the ghostblade

DON'T FORGET TO PICKUP A DEFENSE ITEM AND THE FUROR WHEN YOU NEED EM

Always remember that these items are not to be bought based on my theory only but you may prefer to get them so no hard feelings
Runaan's Hurricane
Lets face it,Apply blight to multiple targets seems so tempting but this is one of the most inefficient items (yeah i might seem biased) varus already is weak at 1v1 and why would we buy and item that only gives you AS? the passive is like only useful while farming in a proper team fight you are to kill the person who dives to you and join your team where this item becomes useless,sure if you are ahead and you are gonna 1v3 this works.also remember that varus encourages the enemy team to split up cause of his ult

Essence Reaver
The same as runaans this item offers mana sustain which varus needs early but by the time you buy this item you are out of that stage so you would rather just buy a few mana potions and besides bloodthister offers so much more,but this when you wanna have a sixth offensive item maybe but for that too there are better choices

Manamune
You aint stacking it,just buy pots

Maw of Malmortius
If you need MR a Banshees is more effiecient or a mercurial schimitar for the cleanse and MS

Sword of the Divine
Lets face it no one buys it

Black Cleaver
if you want flat pen get ghostblade maybe

Trinity Force
This is an item that some claim to be viable I am not gonna argue that it is bad but like every other item on this list it is not as efficient as the gold spent
the sheen procs are not worth it as u would wanna maintain your range as varus and not go in for the sheen proc after you Q
The phage may be good but you would rather get furor than waste a slot for it