When I transform into a werewolf with Vilkas as a follower he attacks me. Why does he do this?

Might be because he is tagged as a guard. He will also attack if you have a high bounty.

This new saddens me deeply

Youll be okay.

I think everyone will attack you when you're a werewolf, right? ...even spouse and followers.

They aren't supposed to attack if they are also a werewolf.
Isn't Vilkas one? Or did he get cured?

Oh that's right I think he did get cured.

The entire companion guild highly disappointed me, not just Vilkas but every one last of them.

I remember being as a werewolf traveling near windhelm and Vilkas started attacking me as well. He wasn't cured at the time. You would figure out that the werewolf is one of your buddies by the scent. I guess not since Skyrim is for the land of the stupid.

Vilkas has a constant sneer and a stick up his butt. I'm not surprised.

This is an oversight on Bethesda part. Or maybe even the companions need to maintain a facade of respectability and cannot turn into werewolves when other people are presents (including members of the circle)? You could use this derp justification to reconcile with the gameworld.

From the wiki:

• Almost all NPCs will become hostile (exceptions include other werewolves, http://www.uesp.net/wiki/Skyrim:Maiq_the_Liar, members of http://www.uesp.net/wiki/Skyrim:Dark_Brotherhood, followers and housecarls, but not your spouse, unless your spouse is one of these)
  
• Being seen transforming by anyone other than the above exceptions adds 1000 gold to your bounty

So a cured Vilkas will attack you.

All Im going to say is that if the next time any of the members of the companions tries attacking me while in werewolf I'm going to play the ole mighty god and use the console to disable their essential status and send them off to hircine (if they're a werewolf) or to sovngarde. I don't care if their an essential. I aint putting up with treachery. They forced me to become werewolf, and now they want to attack me when I am in werewolf form? Besides I'm the boss so it's my way of giving them a good dose of ass kicking, since I never attack them anyways.