Are vacuums tubes suitable for switching high current (~30amps) at high frequencies (~K-Mhz)?
A very short answer with not too much detail goes like this:


Tubes are good at medium to high voltage and low current.
Transistors (especially MOSFETs) are good at low to medium voltage and high current.


Exceptions exist, but for the common and most used examples of said parts, the above... ahem... selection guide is a start.

For fast (>&nbsp;10&nbsp;kHz) switching applications based on tubes, you may want to look into (literature for) horizontal deflection circuits in old (pre-1970) TV sets.