My favorite thread for sewing machine use (not necessarily Viking), is Aurifil. It produces *so* much less lint than other threads I have used for piecing. I use Aurifil 50 wt, and another thing I like about it is that a lot more fits on the bobbin, so I need to change the bobbin less often.
Aurifil is very expensive in my local LQS so I usually look for a sale from an online source. Was really happy with the prices I got last time from
http://www.sewezdesigns.com when she had an Aurifil sale going. Red Rock threads is another good discount source online.