I bought one of these the other day at a comp.fair and installed it ok, also using 98SE. What I did find from the ViPower web site is that there is a little sticker on the USB cable plug which states which version driver you should use. The discs have a number of versions on them. Mine is AT2. Once I sussed that there was no problem.
Your printer may be a USB 2 working on Win98 at USB1.1 unless you put a USB 2 PCI card in the comp if you did not then it is working at the slower speed. Your Drive will not work as Win98 does not support USB 2 unless you have specific USB 2 Drivers that state that it's for Win98