If you use a legit account in conjunction with a steam crack (most probably the undeadpatch) Yes, eventually steam will ban you're legit account. IF however you make 2 steam installation directories, and from the second one rename the main folder and delete you private details such as clientregistry.blob and you Steamapps account folder, you can patch you're second installation and have a cracked account. When you want to play legit, you just run the steam from the original installation.
If you want a taste of online VAC2 servers, I recommend you talk to Campstaff about his "Rage" accounts. He is legit.