Best way to supply heaters for VFD tube
A 400mA buck converter would probably be easiest. As below, alter the resistor values to get your desired nominal filament voltage. Simple, cheap and tiny. 





You could also whack the filaments with the full input voltage (anodes off) at very low duty cycle but that's kind of ugly and if anything sticks on (such as your micro freezing) your VFD filaments will quickly die. At 9V in to get 1.2V RMS at the filaments your duty cycle would be 

\$\frac{t_{ON}}{t_{OFF}+t_{ON}} = \frac{1.2^2}{9^2} \approx 1.5 \% \$