Vienna malt is supposed to be mashed. If you are going to do a mini-mash, I would sub some pale malt, preferably belgian, for a bit of the light DME and mash pretty low so that you will get the attenuation that you want in a saison. All that DME may leave you with a higher than desirable FG for a Saison.
If those are your only available ingredients though, definately still go for it. It will still make a good beer, just a little sweeter and not as dry as you would expect a saison to be.