You can easily smoke a MOSFET if the gate-source voltage is not high enough to put the part fully into an on state. MOSFETs are not digital devices (either off or on). They are analog with a varying resistance over a range of gate-source voltage, more like a valve than a switch. If gate-source voltage is too low and source-drain current too high, the power rating for the part can be exceeded.
For series cells gate-source voltage is always high enough to put a typical MOSFET fully into its on state, but with a single cell or parallel cells you have to select the MOSFET carefully to be sure it's fully on at minimal battery voltage.
Most MOSFETs are designed to be fully on at 4.5 Volts gate-source voltage. MOSFETs with a lower gate-source threshold are available. They're sometimes called "logic level" MOSFETs. They turn on fully with gate-source voltages as low as 3V.
You also have to check the drain-source resistance is low enough when the part is fully turned to be sure the part will not overheat at a desired power output. Not only does the gate-source threshold have to be low enough, but the on-state resistance has to be low enough as well.
When using a single cell or parallel cells not just any ol' MOSFET can do the job. Though with proper selection it's possible to design a switch capable of loads higher than you would ever actually vape with.
