Where should I keep a lager while force-fermenting?

[Skyforger]

I have a vienna lager that's been in primary for 17 days. The last 4 days it's been at about 60-65F, diacetyl rest. Problem is, I'm not getting the attenuation I want out of it. OG was 1.051, was 1.020 at the beginning of the d-rest and is has now been at 1.017 for the past 3 days. I'm planning on doing a forced fermentation of the wort, as I have some of the yeast left over, to see if it's stuck or if I just didn't mash right. It'll take a few days, though, and I'm not quite sure what I should be doing with the batch in the meantime. Should I begin to cool it? Leave it at about 65F? Or should I skip the forced fermentation test altogether and just rack to secondary, begin lagering, and hope for the best? Bear in mind I don't have the equipment to cool it slowly, so beginning lagering will probably crash out the yeast, curtailing any further activity.

edit: I'm using WLP833 (German Bock) yeast, used a 2L starter, and had an expected FG of 1.014-1.012.

 

[Brewpastor]

What was the grain bill and the mash temp? The beer may be done and at 1.017 from a 1.051 on a Vienna lager I would imagine it is just fine.

 

[Skyforger]

The grain bill was:

66% Vienna malt (Weyermanns)
33% Light Munich malt (Muntons)

A scribing error kind of screwed up my planned decoction/step mash. Basically, I had a mash that went like this:
Protein rest @ 122F - 20 mins
Sacc. rest @ 155F - 60 mins, dropped to 147 during this time

Pulled of a decoction of about 1/3 the mash during sacc. rest, and used it to bring to mash-out temps. I boiled down some of the first runnings until they began to caramelize (about 1.5 qts).

so yeah, the mash was a bit convoluted. I had some brief temp fluctuations in there that I cut with ice, I don't think they make much difference though. The caramelized first runnings also add some variables. So I suppose this could be my actual FG. I was expecting it to be a bit high, but not quite this high.

So you think I should just start lagering? Even if so, I might still force ferment a bit of it just out of curiosity.