Thanks for creating OpenMx, a wonderful R package!
According to the manual of version 0.2.3-1006, vech() is supported in mxAlgebra() via MCMCpack while vech() is not listed in http://openmx.psyc.virginia.edu/wiki/matrix-operators-and-functions  . I tried it in version 0.2.3-1006 but it failed. This also applied to other functions, such as diag(), c() and c(t()) (a ")" was missing in the manual).
Here are the example.
> A <- mxMatrix("Symm", nrow = 3, ncol = 3, values=1:6, name = "A")
> B <- mxAlgebra(vech(A), name = "B")
> model <- mxModel("test", A, B)
> fit <- mxRun(model)
Error in substituteOperators(as.list(retval)) :
Could not find function with name vech and 1 arguments
By the way, would it also be possible to implement a similar function of vech() that does not include the diagonals? This function would be useful for analyzing correlation structure. Thanks.