And just for info, since I gained little experience working on tree_branching *.lsp routines...
Be careful in AutoCAD with (vlax-curve-getpointatdist) function... In my case distance was greater than overall length of curve and instead of throwing out nil as a result - output, in AutoCAD result was starting point of curve (in my case it was SPLINE, but that doesn't matter generally speaking)... BricsCAD was here fine and I hadn't have to work to overcome issue... All in all, as I had time, I've cobbled out working both codes in both environments... It's just that something not necessity happen to had to be written...