What was the most common US tank in service by the USMC in Vietnam? I think it's the Patton, but I don't want to screw up!

Roger

M48A3 Patton. It was the only tank in US Army and USMC service in Vietnam. Many other AFVs were used, but no other tanks.

A couple of other M48 variants were used by US forces as well. The gasoline engined M48A2 and M48A2C were used in small numbers.

The M48A2C had many of the fire control and suspension improvements found on the M48A3. One of the visual recognition features when determining an A2 or A2C from an A3 is the lack of the external air cleaners on the rear fenders.

The Marines also used a variant of the M48A3 called the M67A2. The 90mm main gun was replaced by a flame gun.

The M551 Sheridan Armored Reconnaissance/ Airborne Assault Vehicle (AR/AAV) is a light tank that was used by the Army as well. You can debate whether or not this is a true "tank"; however, crewmen are called tankers.

No debate needed, the M551 Sheridan is a tank. Out here.

According to Squadron's M60 in Action by Don Greer, the M60 based M60 AVLB and the M728 CEV both saw action in Vietnam. It further states that there was (at publication time in 1986) an "unsubstatiated report that an M728 destroyrd a North Vietnamese T55 in an engagement [though] little is known about this incident."
As to the Marines, they also used the M67 flame thrower version of the M48. Other armored vehicles used by the Marines, while not tanks, included the M109 and M110 SP artilery, the Ontos SP recoilless rifle, LVTP-5, LVTE, LVTH, M116 amphbious personel carriers, m733 carrier, , M51 Tank retriever, M 76 Otter, and the 155 MM sp Howitzer vehicle. (According to Armor in Vietnam by Jim Mesko)