As I recall from Susskind's course, there is no actual vacuum in string theory. There are some pieces of information, which can be helpful, like terminology developed for 2 decades.
Please, note the dates.

String theory is believed to have a huge number of vacua — the so-called string theory landscape.

Terminology starting from almost nothing:

*"In discussing compactifications of string theory, we will discuss only vacuum states that can be described as the propagation of strings
in a background space-time. It is quite conceivable that more complex,
"inherently stringy" vacuum states should be considered, but a
workable approach to considering them ***does not appear to exist** at
present."

Candelas, P., Horowitz, G. T., Strominger, A., and Witten, E.Vacuum congurations for superstrings. Nucl. Phys., B258, 46. 1985

Developed to more specific ideas:

*"The vacuum structure of the theory, called the string theory landscape (or the anthropic portion of string theory vacua), ***is not
well understood**. String theory contains an infinite number of
distinct meta-stable vacua, and perhaps 10520 of these or more
correspond to a universe roughly similar to ours — with four
dimensions, a high planck scale, gauge groups, and chiral fermions.
Each of these corresponds to a different possible universe, with a
different collection of particles and forces",

de Sitter Vacua in String Theory, by Shamit Kachru, Renata Kallosh, Andrei Linde, Sandip P. Trivedi, 2003.

*"Flux compactications typically give very many possible vacua, since the fluxes can take many difrent discrete values, and there is no
known criterion for choosing among them. These vacua can be regarded
as extrema of some potential, which describes the ***string theory
landscape**."

String Theory and M-Theory: A Modern Introduction, by Katrine Becker, Melanie Becker, 2007, p.477

*"Gukov, Vafa and Witten (2001) made it evident that Flux compactications can lead to a solution of the moduli-space problem,
since a nonvanishing potential for the moduli elds is generated. This
led to the introduction of the string theory landscape, which
describes a huge number of ***possible string theory vacua**, in
Susskind (2003)."

Gukov, S., Vafa, C., and Witten, E. (2001). CFT's from Calabi{Yau four-folds. Nucl. Phys., B584, 69. Erratum { ibid. B608, 477. E-print
hep-th/9906070.

Susskind, L. (2003). The anthropic landscape of string theory. E-print hep-th/0302219.

String Theory and M-Theory: A Modern Introduction, by Katrine Becker, Melanie Becker, 2007, p.715

Can't find any other notions dated after 2007. Hope this was helpful.

This post imported from StackExchange Physics at 2014-04-04 16:40 (UCT), posted by SE-user Sigrlami