Splines = BAD
Polylines = Good
I use polylines exclusively and Vectric's software works 100% of the time with my drawings.
Say you have a rectangular shape, and you want to start in the middle of the edge.
Here's what I do.
First, draw the rectangle. This will be used as a base for the final part.
Using the polyline tool, use a Midpoint or Nearest snap at the point you want to start. Then, click on the endpoints in the order (direction) you want the tool to go (Use running OSNAPS with Endpoint turned on). After clicking the last endpoint, hit "Enter" to exit the command. Now, delete the original rectangle, which should be visible in the open section of the new , open, rectangle. Then, select the new one, right click and choose polyline edit, and choose close.
Save as .v12 dxf and you should be good to go.
I actually use a macro that I wrote which exports g-code from within AutoCAD.