The illustration on the left shows a quick-and-dirty way to at least start a slot in the end of a piece of round stock, in this case, a piece of K&S 1/16 inch brass tubing. 

If sawing tubing, pick a saw blade with very fine teeth.  The K&S 1/16 inch brass tubing shown has a 1/32 inch wall, so the saw blade should have at least 64 teeth per inch. 

Position the stock as close to the center of the jaws as possible, and so the end is below the top of the vise jaws.  Then hold the saw in such a way as to contact opposite corners of the vise jaws. 

In theory, holding the saw against the jaws will guide the saw so the the slot will pass through the centerline of the stock. 

In practice, the set of the saw teeth and other factors make it is difficult to hold the saw blade exactly in line with the stock, and the slot will start to wander. 

If this is a problem, you can use this technique just to start the slot.  When the slot is deep enough to guide the blade, transfer the round stock from the vise to a vise with a V-block, or perhaps to a drill chuck*, which will hold it more securly than unaided vise jaws could.  Then examine the slot to see which way it's off and continue the cut by hand, guiding the blade toward the centerline of the stock. 

*Just don't turn on the drill!