Cheater.....
Or not, but I dont rly believe that your brother, friend, girlfriend (boyfriend maybe), spammer or friend from a clan hijacked ur steam. I think you though: "Hmm I never tried playing CSS with wallhack, what would THAT BE?!". And so did you get banned.
Maybe its not true, maybe your cat played on your account, but VAC ban isnt given out, couse steam crew was bored, had nothing to do, "so lets give him a VAC ban I dunt like him lawls lololol".
And btw, I never heard of anyones account being hijacked, if you DIDN'T GIVE OUT YOUR PASSWORD!