Here's one of our favorite Vette sites biggurnz. It will tell you how to decipher your VIN# to find out lots of information about your 1981
http://www.100megsfree4.com/corvette/1980/vet81.htm
As for your second question, the type of transmission you have in the car doesn't really change the HP output of the engine which in your vehicle is 190 HP. The change lies in the gearing of the transmission and the points at which you change gears. That will affect the acceleration of the car and it's top end speed although it is unwise for you to explore the latter attribute too extensively. The question is more "Does it make a difference to you, or the way you drive?" Unless you are street dragging I'm guessing no. If you are a guy who loves a stick then this might sap your enjoyment a bit but only you can answer how big a problem that is. An automatic transmission will of course dull some of the more high-performance capabilities of the Vette but that's unlikely to impact you on a day-to-day basis unless you are racing it. Based on your concerns about it's value I doubt you are.
Oh and perhaps a Vette guy can tell you differently but I've found no evidence that plant in which the car was made has an impact on it's value. But here's a great site on things to factor into your cars value.
http://www.geocities.com/MotorCity/Downs/3000/guide2.html
Hope that helps!