Vacuum tube LED flasher
hmmmm.  Speaking in general, there are plenty of tube oscillator circuits which would work nicely for a flasher; however, I'd suggest that a dual triode based multivibrator would be a logical choice.  Depending on the current requirements for the LED, you could put one in series with either of the plate resistors.  You'd need an LED that only needs a few mA to light so that it will turn on with the limited plate current.

In terms of using the parts you have, I see a few challenges:


you've got 3 pentodes (12au6, 12ba6) and a dual diode + triode (14gt8).  (not listed in this question, but in one of your other questions, the 12DT8 is a dual triode and might work).
Tube circuits normally have 300+ Vdc for their plate power supply and your supplies are no where near this voltage level.


While there are solutions to the above challenges, I'm going to assume you will want to make your flasher circuit work with basically only using the parts you have available /  described in your question.  That is, the tubes, the power supplies, 555 timers, small signal transistors and the assorted R's and C's one would use with these semiconductors.

Thus, my suggestion would be to build an LED flasher using the 555 chip and power it via the 20Vdc power supply connected in series with one of your vacuum tube's heaters.  You will need to place a resistor between the heater and ground to assure a minimum current through the heater.  Put your LED flasher circuit in parallel with the resistor.
I'd suggest something around 80 ohms and greater than 2W. If you don't have any 2W or greater resistors of the right value, you can do series/parallel combinations of lower rated resistors which solve to ~80 ohms.  (remember Rseries=R+R+R+R, 1/Rparallel=1/R+1/r+1/R)

While the tube is not functionally active as an oscillator, it does provide the key functional operation of dropping enough voltage to reduce the 20Vdc supply to be within the 555's limits...

Note the opportunities to improve this circuit abound.  (It doesn't even have a PIC 10F200! :) )