just started sc2 as got 2 weeks off and some time at last! Doing quite well but have problems late game vs mass air as toss espcially the banshee vikings with T or infact any air massed? is mass air unbeatable unless you mass air yourself

or is fact got lock them down a lot earlier
This is tough, while phoenixes in theory soft counter vikings and should give protoss air superiority vikings are seldom deployed by themselves and with their range give airspace dominance over a ground army of stimming marines and tanks.
If he's pushing out with banshees and vikings without this ground support then phoenixes should at least limit their mobility and force them to move out with ground support. With with ground support, really, terran air becomes dominant and nigh on impossible to fight cost for cost.
I would suggest building 5-6 phoenixes (and no more!) a couple void rays a load of stalkers with blink and HTs. Use HT feedback on battle cruisers, medivacs and banshees. Storm the **** out of marines and banshees and try to engage the terran as close to _his_ base as possible.
The main strategy when encountering this sort of strategy is to affect a contain on the Terran and try to get 3-4 bases up yourself (with double pylon, double/ triple cannons in a triangle formation around your nexus'.
If you can pin him to 1-2 bases you can afford these defensive structures to to shutdown any harass. Keep observers everywhere and hot key them. Don't push into his base too much keep them at the borders of his base and if one gets sniped always build another one. Keep them in your base as well in case some banshees sneak in.
While he's turtling just use a couple warp prisms to drop a couple zealots in his main while dropping 4 hts in his expo (if he has one). 4 hts in the prism then warp in as many zealots as you can. While he's chasing your 2 zealots in his main, send your 4 temps to storm his expo line (which should obliterate it completely) then complete the damage with your zealots. Then merge the hts to archons and focus kill his refineries and any left over scvs. You're throwing these units away, but try to deal the most economic damage you can. If you can take out his cc go for it. But taking out the refineries is usually quicker and slows him down letting you get 3 and 4 bases up.
Keep harassing as much as you can but mainly focus on getting those 4 bases up getting 10-12 gateways and a couple stargates along with upgrades. Blink/charge, and especially +3 attack, +1 armor.
This is a game dominated by econ in the end. If the Terran player gets an econ advantage you lose. You have to win this zerg style but always having more money and more units than the terran player. At somepoint he will do a mass push, usually mech based 5-8 tanks, a thor or 3, banshees and a million marines and medivacs. Catch this ON THE MOVE. Feedback in this order medivacs, thors, banshees, then focus storm the marines and banshees.
You'll still lose the fight, but by engaging far away from your base you can warp in stalkers and zealots to clean up. From there you should win.