Yes! You can, and should try VueScan before you purchase. Make sure it works with your scanner before purchasing.
You can use the version you purchase for as long as you want. You get free upgrades for one year with the Standard Edition and unlimited free upgrades with the Professional Edition.
There is usually an upgraded version of VueScan every week or two, and these include improvements, new features, support for additional scanners, bug fixes, and support for new operating systems like Windows 8.1 and Mac OS X 10.10 (Yosemite).
You can use VueScan on up to four different computers that you personally use, with any combination of operating systems, with any number of scanners, both x32 and x64, with a single license.
After a few seconds, you will get a printable receipt in your web browser that contains a VueScan serial number and customer number.
Enter these numbers into the trial version of VueScan, and this will enable the features for the Edition you've purchased.