@JCodgell
I'm not understand everything what you say. I'm relative new in Vectorworks after have WYG over 20 years...
Now I created a new blank document from Spotlight Metric tmp. select from Vectorworks library the Prolyte H40V-300 truss. placed on the plot. to connect the second truss I can connect only 1 side of the truss. finish truss tool, select this truss, right click and choose "Customize truss symbol Data" then arrive to truss Properties - Is this the Braceworks Data? so than in the Straight Truss tab looks like everything correct but doesn't. I click to "change cross section" inside Cross section Data, come up the pop up window. the data shows everything fine. Prolyte H40V details. press OK. than Press OK on the truss properties. than my symbol work!!! if I check in ISO view I see the cross on both end which was empty before. doing the same on most H40V items. I think except H40V-250 what I found on this test. The Box corner is different because when I opened this Truss Properties on the same way under the custom geometry all x,y,z data is 0 so here I must to change the details to than press OK. than the symbol work.
if I use my metric meter template -was created a standard metric template of VW just changed the units than saved for temp.-, when I access the same truss property windows (hopefully Braceworks) the H40V-300 showing at length 3000. I must put behind the "mm" measure than change and showing 3 instead of earlier 3000.
I attached my test file created from Spotlight Metric include trusses. the 1m and 1.5 meter section was not modified so doesn't worked. Fixture test.vwx I also attached a new test for you without any editing. you can see just the 2.5m section able to connect normally on both side you also can see on the cube that cross data also missing.
Regarding with the circle I created as you wrote with the circle truss tools than created a symbol than convert to truss, than setup circle truss properties. it look good but doesntVW support test.vwx attached this test include my circle..thanks for your help!