Yes, we're weird. Most of these, you'll notice, are by Sarah McLachlan. So sue us. ... ...Don't really.

Vash, or Vash to Meryl
"Bachelorette", by Björk
"Sweet Surrender", by Sarah McLachlan
"Black & White", by Sarah M.
"Alarm Call", Björk
"Human Behavior", Björk
"Porcelina of the Vast Oceans," by the Smashing Pumpkins

Meryl, Meryl to Vash, or just Vash/Meryl
"Iris", by the Goo Goo Dolls
"Do What You Have to Do," by Sarah McLachlan
"I Love You", by Sarah M.
"Vox", Sarah M.
"Call Me Call Me", from Cowboy Bebop (Yoko Kanno)
"In My Life", The Beatles
"True Colors", Cyndi Lauper

Wolfwood
Big Bluff (Trigun BGM)
Blue Funk (Trigun BGM)
Never Could Have Been Worse (Trigun BGM)
"Vincent", Don McLean
"A Long December", Counting Crows
"The Unforgiven II", Metallica
"Low Man's Lyric", Metallica
"The Outlaw Torn," Metallica [note, this could also be Vash...]
"Rain", from Cowboy Bebop

Wolfwood/Milly
"Full of Grace", Sarah M.
"Thompson Girl", The Tragically Hip
"My Lover's Gone", Dido
"Building a Mystery", Sarah M.
"I'll Be There", Escape Club
"Time in a Bottle", Jim Croce

The Marijuana Song - Legato (Heh heh heh...)

Suggested (thankyoooou!) by Cadence:

VxM
"Learning How to Smile", Everclear
"Unemployed Boyfriend", Everclear
"Good For You", Third Eye Blind
"Slow Like Honey", Fiona Apple
"Solace", Fuel

Knives
"Spider in My Room", Barenaked Ladies
"Knives", Fuel
"Prove", Fuel

Vash
"Burn", Matchbox Twenty
"Hemorrage", Fuel
"Innocent", Fuel

WxM
"Dandy Life", Collective Soul
"In The Drink", Barenaked Ladies

Legato
"Low Place Like Home", SneakerPimps
"Easy", Fuel

Suggested (YAAAY!) by Musashi:

Vash
"Albuquerque", by Weird Al Yankovic
"Kryptonite", Three Doors Down
"Desert Rose", Sting

Wolfywood
"Losing My Religion," R.E.M.

Knives
"My Evil Twin"