You don't have to worry about your cash getting seized, you have to worry about it getting STOLEN. Make your package look like any other package, tape it shut securely and neatly, and there should be no issues.
You don't need to vacuum seal it but I would suggest putting it in some sort of barrier to conceal it in any situation. The only packages that are sniffed out are ones that are obviously sketchy looking, reaking odor, or leaking liquid - or that have been tipped off. They don't put the dog on the belt to sniff all packages and they don't X-ray all packages. XRaying would take too long. Dogs work only by them isolating the dog with the package and if he alerts on it then they will open it. Since there is drug residue everywhere, dogs will alert on just about anything, so your best bet is to make the outside of your package look as normal as possible.
And NEVER send cash in First-Class mail. Use Priority or express only. First-Class gets 'lost' in transit too easly.