Yes, you should hold Vanco until the trough level is known, as those numbers are essential for the proper dosage to be calculated. I know LTC is notorious for late lab draws (and other less-than-optimal aspects of sub-acute care), but those peaks & troughs, plus daily weights, are necessary in order to give therapeutic doses.....otherwise, what's the point of giving it?
Also, to answer your PICC question: I personally have never, nor would I ever advise anyone to use a vacutainer to obtain blood through a PICC. The catheter itself is rather delicate, and too much pressure of any kind can easily fracture it. This is one task you really should take your time in performing, not only to avoid damage to the catheter but to prevent lysis of the blood cells. There are several different techniques, you have to experiment to find out which works best for you and the particular patient situation you're presented with.