I'm not sure what you're trying to do there. Putting a test in a return statement is likely to mess up the precedence of how things are run, you may be thinking "if vX, return either eX+=o_dx or eX-=o_dx" (which in itself is nasty, since youre relying on the (a+=b) returning a result equal to a+b), but the compiler might see "if (return vX) then add o_dx to eX or subtract o_dx from eX" which doesn't really make sense, but is possible, depending on the precedence used in the interpreter/compiler.

?: is one of those operators that has very limited good use, and I don't think your example is it.

If however you're still wanting to use it, I recommend adding lots of brackets to avoid any ambiguity in what you mean, e.g.:
return ( (vX) ? (ex+=o_dx) : (ex-=o_dx) );

Personally I'd do:
if(vX)
 ex+=o_dx;
else
 ex-=o_dx;
return ex;