(Adding a bit of general licensing knowledge to benefit the site)
There is a lot of confusion in this, I recently asked my rep at Microsoft and was told along the following...
Windows Vista with free upgrades - Purchase Windows Vista now and get a free upgrade to 7 later... Available to all versions excluding home basic (as this is being phased out for mainstream markets).
There are two sorts of upgrades available -
Anytime Upgrade which if you own Vista Home Basic, you can go up to Ultimate... If you own 7 Home Premium, you can go up to Ultimate e.t.c. The other sort of upgrade is what you are talking about -
Windows 7 Upgrade editions on the hand, you simply purchase the edition you want and as long as you have a previous licence, you have the "right" to install it. Feel free to buy any Windows 7 upgrade package, and as long as you have Vista, you can use it.
As for actual feasibility, Windows Vista and 7 for upgrade actually require the OS to be installed. There are a few workarounds such as installing without a serial key so it thinks you have a fully packaged product and then installing over the top as a upgrade, however as much as many people have had success, I would not recommend it, unless you are sure you know you want to do it.
I am still not in favour of upgrading in place, I am always for new installations, but I know on Vista and 7, it is a lot better, and many people have had success, so Install Vista, then install your 7.
Personally, I would never upgrade from a Beta, RC, or aything other than RTM (Final Edtion).
To sum up - 1, Yes, you can buy a upgrade and use it... 2, Yes, install Vista (preferred) or use the method above.