The HTC Vivid uses the standard GSM SIM. It does not use a microSIM. For LTE, I have an old SIM that I used for 3G, but I'm still able to get 4G, HSPA, HSPA+, and LTE. The age of the SIM has nothing to do with service. The size of the SIM matters.
If you got a new sim with the phone you don't need a new one for LTE service. However if you kept your old one take a look at the SIM serial number (ICCID) and if it has a 25 about halfway through you should be set. If not you may want to request a replacement from AT&T... they are free in corporate stores.
Sent from my Motorola Droid RAZR Max HD using the Android Central Forums app.