If this saw is capable of miter cuts, it should be able to be adjusted for squareness relative to the vise around a vertical axis. If the locking detente (at 90°) is out, you might have to fiddle with that. Check the squareness of the cut on the end of the tube after taking a cut. It might be necessary to adjust the height of your support stands to ensure a square cut.
Indicate the saw blade for true running around the spindle, just to check the condition of the arbor flange. If there is no wobble ( minimal wobble) to the blade you will at least then be able to use a precision square against the blade to check vise squareness. Blades are usually hollow ground, so allow for that. If you have a large enough precision square, you can remove the guard off the saw, and check for simultaneous square contact at both top and bottom or front and rear of the blade.
Thin wall tubing like that may be reasonably tough and could easily damage the sharp corners of the blade teeth. If the blade gets dulled, it will wander very easily. Make sure the rpm is correct for the blade diameter used, and use flood coolant for stainless.
They usually recommend that 2 or 3 teeth be in the cut at a time, so you need to have a suitable pitch blade available for the wall thickness of the cut. While the saw will cut thin wall very rapidly after it breaks through the top, the feedrate should be moderate for the downfeed as it seems to me when I really 'push' the saw, is when I get wandering in the cut.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)