The cheapest option is a USB -> DMX interface. These are available commercially, though I get by with a USB to RS485 board which works fine for me. Some lighting controllers which connect over Ethernet also offer DMX outputs (as well as WS2811 for pixels)
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More elaborate commercial units will offer electrical isolation etc.
If you have connected DMX dimmer(s), just add each channel as a single colour node. The most relevant effects in Vixen are 'set level' and 'pulse'. Other effects (such as twinkle) can also be used depending on how you set up your lights and their groupings.
Important concept - a node (DMX channel = single colour light) can be in more than one group.
You can control any type of DMX fixture since ultimately Vixen just outputs bytes of data on the channels you have configured. I believe better support for things like moving head fixtures is being added (as in making them easier to program in the Vixen environment).
I use Vixen to control a mixture of DMX and WS2811 in my garden / Christmas show