  We verify that the dilaton together with one exactly marginal field, form a
moduli space of marginal deformations of closed bosonic string field theory to
polynomial order five. We use the results of this successful check in order to
find the best functional form of a fit of quintic amplitudes. We then use this
fit in order to accurately compute the tachyon and dilaton effective potential
in the limit of infinite level. We observe that to order four, the effective
potential gives unexpectedly accurate results for the vacuum. We are thus led
to conjecture that the effective potential, to a given order, is a good
approximation to the whole potential including all interactions from the
vertices up to this order from the untruncated string field. We then go on and
compute the effective potential to order five. We analyze its vacuum structure
and find that it has several saddle points, including the Yang-Zwiebach vacuum,
but also a local minimum. We discuss the possible physical meanings of these
vacua.
