Thanks Lars for your infos.
I think that the several ways one can deal with light signals from viessmann are so great that i thought on making a sum of all things said before... here it goes.
For what i found out from Viessmann webpage and from the vast knowledge of this forum, i can see the following five solutions that can:
1) Control Viessmann light signals (i.e 4011)
2) Control track current
3) Work with digital control The five possibilities are (all references from Viessmann):
1) 5551 + 1/4 of 5211 (k83)
2) 1/2 of 5552 + 1/4 of 5211 (k83)
3) 5221 + 1/4 of 5211 (k83)
4) 5224 + 5228 (the new "Viessmann way")
5) 5213 + 5213 (k84) (one for track current, other for signal)
One thing is usually important: costs
So let's look at some numbers based on Lokshop prices.
(it's not my dealer, but it's a well known one

)
5211 - 27,65 €
5213 - 43,26 €
5221 - 16,52 €
5224 - 32,00 €
5228 - 9,60 €
5551 - 11,69 €
5552 - 17,80 €
5214 Powermodule - 11,96 €
(this one is an option for powering decoders like 5211).
Let's start adding some numbers [:I]
NOTE: This configurations is for
controling track current and for a digital system.
1)
5551 (11,69) + 1/4 of 5211 (6,91)
Total: 18,60 / each signal (w/out signal!)
Gives a 4-pole relay (if you have working catenary or ...)
2)
1/2 of 5552 (8,9) + 1/4 of 5211 (6,91) Total: 15,81 / each signal (w/out signal!)
Gives a 2-pole relay (one for signal, one for track current)
3)
5221 (16,52) + 1/4 of 5211 (6,91) Total: 23,43 / each signal (w/out signal!)
Gives soft light change (a good improvement... like the new digital Marklin signals)
4)
5224 (32,00) + 5228 (9,60) Total: 41,60 / each signal (w/out signal!)
Gives you soft light change and a signal bus, very good for a full signalling system.
5)
1/4 of 5213 (10,81) + 1/4 of 5213 (10,81) Total: 21,63 / each signal (w/out signal!)
If someone can tell us that only one k84 makes it, please say so! Viessmann does say it needs one for for track current and other one for the signal itself.
Hope that this can be as usefull to others like it was to me

Thanks for all your help.