The Linpus Linux Lite V1.0.21.E. Operating system is a commercial version of Linux customised for use on ACER Aspires etc. Because it is a commercial version of linux, it is not as easy to update or mess about with and updates are not as frequent as with main Linux Distributions such as Ubuntu.
The kernel...(the core of Linux)... is apparently an old one and it may be that the version installed on your machine does not have the included code necessary to run a 3G dongle.
However, there are various ways to possibly sort this out:
1) It may be that the dongle may work on the machine, but that the means of configuration are somewhat unclear. It maybe that a linux nerd could configure it without to much hassle as I understand that Linplus is based on the Fedora Distribution.
2) It may mean updating your machine's Linux kernel through the Asus online software support facility and then installing software.
3) Alternatively, there was a Linux dialer developed by some vodafone nerds, a couple of years ago. It worked on an earlier version of Ubuntu perfectly.
4) If all else fails, I would recommend reinstalling the machine with Linux Ubuntu Netbook Remix which is specially configured for Notebooks. I reinstalled an Acer Aspire One with this package a couple of months back and it has been a very stable system.
It may also be necessary to boost memory to 1GB for a dongle ot work properly.
One thing is sure make sure you get someone who knows Linux to sort it out. Most computers shops/Vodafone here in the Algarve do not have the necessary experience/knowledge.
However, I would recommend you get the machine tested and working with a 3G dongle prior to buying one.