Code:
...
   var lerped_z = (1-LerpAmount)*player.z + LerpAmount*other_z;
   vec_set(camera.x, vector(player.x, player.y, lerped_z));
...

In Lerped_z formula, you many need to swap player.z and other.z around to get what you want...

Or, another way is this
Code:
...
   VECTOR tempV;
   vec_lerp(tempV, player.x, vector(player.x, player.y, other_z), LerpAmount);
   vec_set(camera.x, tempV);
...


NOTE, neither of these snippets have been tested...

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A8.30.5 Commercial