My understanding is that you need to make a specific agreement. So submitting an order definitely makes you guilty of attempt to purchase a controlled substance. Telling a vendor you are transferring bitcoins and will submit your order tomorrow probably makes you guilty of conspiracy to possess a controlled substance. Telling a vendor that you want to purchase from them, or will do so "in the future", but it's an unspecified time frame, probably means you can't be prosecuted, or they would have a hard time doing it, so they wouldn't try.