I'm new to this forum and haven't used the ABL scripts yet, but I have some general programming knowledge and any interpreter woud read your current line as this:
IsWithinArea(VongL1a, -6976.0, -320.0, 0.0, 50.0) : true
IsWithinArea(UnitID, position Point, real Radius) : boolean
where the variables are read like this:
UnitID = VongL1a
position Point (expects array) = -6976,0
real Radius = -320.0
because of the way the commas are used
you could try something like:
IsWithinArea(VongL1a, (-6976.0, -320.0, 0.0), 50.0) : true
either way, the function expects an array (however the syntax for arrays is in this language) between the first and second comma within the parantheses