I am also on venlafaxine (Effexor) and I can say that it will probably dampen (but in no way prevent) the overall trip. That is, you will likely have a higher tolerance for the effects than most people because of venlafaxine and will require a higher dose. As this shall be your first time, I wouldn't go nuts with the dose, but be aware that you might need a little bit more than others to really "break through."
Also, don't worry too much about having 'terrifying hallucinations.' Contrary to media portrayals, genuine de novo hallucinations (seeing/interacting with things that are not even real objects) are rare with classical serotonergic psychedelics. You are far more likely to experience an "alteration/enhancement of normal visual perception," as if someone had turned the gain of your visual system way up. The world will still be the same, but you will perceive it with much greater contrast, higher color saturation, glowing edges around objects and translucent, fractal-like colored textures on surfaces. Your perception of objects will be different, so you may think certain things appear different than they actually are, but it is extremely rare to have paranoid/delusional type hallucinations (unless you have a history of schizophrenia, in which case you should avoid psychedelics all together).
I'm with everyone else on the idea of pulling the escape cord during a trip (that is, benzoing your trip to death)--only do this in the event of an emergency. If you do it just to 'come down,' your trip will not be as rewarding or psychically beneficial. Have fun, stay in a safe environment and we'll see you on the other side.