Arguments

curveEnt  (ename or vla-object) the COM object of any "curve" entity
param  (integer or double)  the "parameter" on the curve

Return

double;  the distance of the curve (relative to the curve's start parameter) or NIL

Example

(setq vlaLine (vlax-ename->vla-object (entlast)))  #<VLA-OBJECT IAcadCircle 000000003A012150>
(vlax-curve-getdistatparam vlaLine 0.75)  188.669747018779

Remarks

the "distance" is a relative length value, for some curve types !
to obtain the real distance, in such cases, you need to determine the point at the parameter, and use (vlax-curve-getdistatpoint)