You have full current going through your fire switch. The traces and fire switch are likely not rated for the current load. The switch is switching the current before the mosfet gets a chance to like it should. The fire switch should be connected to the trace going to the mosfet gate. All of your traces for the atomizer, batteries, drain and source connections need to be the proper width for the maximum current load. Use a trace width calculator to determine that.
You would want your fuses to be between the battery+ and 510+. I'm not aware of any resettable fuses with a 9 amp hold that are available in a 1812 package. Do you have a link or part# for the fuse? They might be quick blow fuses, once they trip they're done for.
For your traces, you have quite a few right angle bends. You should be using 45 degree bends instead. The trace for your resistor going to the GND pad & Source pin comes to a perpendicular/right angle intersection. Create a chamfer there such as shown here
http://en.wikipedia.org/wiki/Chamfer#In_printed_circuit_board_.28PCB.29_design. Actually I would just move the resistor to the right so that the right pad is in the middle of the trace, eliminating that intersection all together. Also position components so that trace lengths are shorter.
For your resistor, 2512 is pretty damn big. Try maybe a 1210 or 1206 instead. Although the choice is yours.
You have a power plain on the second layer of your board but it isn't connected to anything. It looks like you plan on securing the drain tab to the board with a screw. I would get rid of that mount. Place pad directly under the drain tab to solder to. You can connect that top pad to that bottom plain you have by placing multiple vias
http://en.wikipedia.org/wiki/Via_%28electronics%29. All of that copper will act as a heatsink for your drain tab. Any traces are pads you currently have going to the drain pin, place close to and connect to the drain tab instead (such as 510- and VM-). You can still connect the drain pin to the board put only to secure it.
Consider a surface mount n-fet instead. A properly selected one will have better specs and be much smaller in size. I use this on most of my boards and am able to hand solder it:
http://www.mouser.com/ProductDetail/NXP-Semiconductors/PSMN1R1-25YLC115/?qs=sGAEpiMZZMshyDBzk1%2fWi%252bFHavO7hMd5ZCHgcLfhazw%3dAlso your board is dimensionally quite large. Can't imagine it fitting into a decent sized enclosure.