Another consideration- 134a will need new barrier hoses to prevent leaks.
If you dont mind a leaky compressor and wrong hoses , you will be ok.Just
recharge every year till the compressor cant take the pressures anymore. 50
feet of barrier hoses , using your old fittings will cost about
$350.Commparison of the two molicules is that the R_12 molecul(sp-again!)
is like a beach ball , and a 134a molicule is like a golf ball.It will
migrate through the rubber.Being enviornmentaly correct with vanagon a/c is
difficult any way you go, unless money is of no concern.It may actually be
much cheaper to go r-12 is many cases.