While checking my first stage I noticed the rubber plug is gone.

[herman]

Take it to a dive shop so they can look at it first hand.
That plug is an exhaust port and if it is compromised you can have a serious leak on the first stage.
That port is what dictates how much air your first stage is giving you at depth. If it bursts, you lose all your IP and you're sucking out of a straw at depth.

Your description but lack of air leaks is conflicting info. I wouldn't trust anyone's advice here unless you posted a detailed photo.
Even then we can't do air checks and feel checks from our arm chairs.

That is the enviromental seal (air bleed) port, which should leak air. The plug simply keeps water from entering the spring chamber but at the same time allowing air to escape. Diving without the plug will not/can not cause a major issue, at least on that dive,left unserviced afterwards is a different matter. At worst it will allow water into the spring chamber which will require the reg to be serviced but otherwise there will be no damage or cause a safety issue. It has nothing to do with "how much air your first stage is giving you" but it does control the regs depth compensation. That said, even with the port completely missing, the reg will continue to depth compensate normally, just as every other piston regulator does. With it gone, it just becomes another unsealed piston reg, just like dozens of others.
OP, I would not suggest you not dive it with the plug missing since that will allow water into the spring chamber. In and of itself that will not do damage or be a safety issue (unless it has already been dove that way and corrosion has already started) but it will require that the reg be serviced very soon or you risk the chance of damage to the piston and/or the body.

 

[kwinter]

Exactly what Herman said. Especially if salt water diving, the reg will have to be serviced.