I am interested in finding out what is known about the following generalization of balanced incomplete block designs (BIBDs): 


"What is the maximum size of a collection $B$ of $v$-dimensional *unit* real vectors with the following property: there exists a constant $\lambda$ such that $\forall x,y\in B$: $x\ne y \implies x\cdot y = \lambda$?"


Such a collection can be derived from a BIBD, but I wonder if larger collections exist?



Vector version of balanced incomplete block designs
The Gram matrix of your vectors is equal to $(1-\lambda)I +\lambda J$
(where $J$ is the all-ones matrix). If $\lambda\ne -1/(v-1)$, this matrix is invertible, whence your set of vectors is linearly independent and there are at most $v$ of them. If $\lambda = -1/(v-1)$, then the Gram matrix has rank $v-1$ and your vectors are the $v+1$ vertices of a regular simplex.


The analogy to design theory is not compelling.


