It can be an arduous task to cut out the seatpost especially when there is a lot of it there but if everything else fails this is the best and least destructive option.
If one penetrant does not work another might... PB Blaster gets rave reviews but ammonia and WD40 also work.
I don't use saddles to provide leverage as the rails often can't take the stress... a bar end can usually be clamped down to to a seatpost to provide leverage and you can also drill straight across the seat tube so that you can insert a small but stiff bar.
With this you can try and remove it while the bike is in he stand or on the floor or can provided a stop for when you put it in the vise and turn the frame.
The last post I had to remove was on my Ron Cooper and if there was a bike that one wanted to save, this would have been it.
Soaking it yielding no good results and when out into the vice the seat post pretty much crumbled so it was 3 hours of using the saw to cut it out...
Gave it a few little taps from above with the hammer and had it drop right into the frame but was okay with this as the club shaped cutout in the bb provided a spot for me to tap it out from the bottom.
And then there was much cheering and dancing in the shop.