Yo richs050, Welcome to SMF, you found the smokiest meat smokin forum on the internet.
Your Cajun Injector Smoker is also known as a MES "Masterbuilt Electric Smokehouse/or Smoker" a very popular smoker here at SMF. Congratulations on your choice of a fine smoker. As for the top vent, it is the consensus of the majority of MES owners to leave the vent wide open. The unit is designed around a very specific amount of inlet air flow for combustion of the wood chips, provided by three small holes in the wood chip tray loader, plus some additional leakage via same loader. Your goal is to get just enough smoke or TBS (thin blue smoke), that imparts the right flavor to your meat. To achieve TBS you adjust the amount of wood chips burning not the air flow. Heavy white or dark smoke is NOT good and imparts bitter flavors to the meat. Depending on the type of wood your chips are, TBS when achieved is easy to identify not only by the light thin blue smoke but also it has a almost sweet smell, at least with hickory and almond wood chips.