[StilThere]

The mistake is here

pawn Код:

format(string, sizeof(string), "MPH: %i", vSpeed);

use %f to format a float

Another little remark

pawn Код:

if(C < 0.0) C = -C; // 0.0 can also be just 0

Don't forget commas

pawn Код:

new Float:CurPos[4], Float:OldPos[4];

And after all I don't get how adding up the distance changes would lead to your velocity, I'd rather do it like this

pawn Код:

new Float:VehVelocity[4];

GetPlayerVelocity(playerid, VehVelocity[0], VehVelocity[1], VehVelocity[2]);
VehVelocity[3] = floatsqroot(VehVelocity[0] * VehVelocity[0] + VehVelocity[1] * VehVelocity[1] + VehVelocity[2] * VehVelocity[2])