I have the interchangeable Viking walking foot and LOVE IT......Being able to to have different feet for for stitch in the ditch etc is wonderful. Plus it works great with any of the feet. You can go to the Viking website or maybe just google Viking interchangeable walking foot and there is a chart that tells you what machines it will fit. Mine fits my Designer 1 and the 875 along with my 186.
They are pricey but I kept searching Ebay and found a new one $40 cheaper that was new and from a dealer. One great place is ShopJoya.com...Go to their website and search for Viking interchangeable walking foot. They have the item and also a list of the machines it fits. Not sure their price is the lowest but much lower than my local store. I found it was worth the money.