Most Anderson shows have a vocal theme of some sort. Wasn't there one planned for THUNDERBIRDS that wasn't used?

If you can believe it, yes. There were actually plans to replace the reprise of the THUNDERBIRDS march over the end credits with a song called "Flying High". Cooler heads prevailed at the last minute and the song was not used during the credits. It does, however, make an appearance in the series' final episode, "Ricochet", on the pirate radio station run by deejay Rick O'Shea.

The lyrics to "Flying High", as heard in the episode "Ricochet", are as follows:

When you left me last night
I was low as a lead balloon
But when you rang me this morning
My heart jumped over the moon

So now I'm flying high
Just like those Thunderbirds men
Flying high
Now that we're together again

When you are not in my arms
I'm a long, long way from Heaven
But now you say that you love me
My heart is doing Mach Seven

So now I'm flying high
It's so wonderful to be alive
Flying high
Just as high as Thunderbird 5