  31P nuclear and muon spin-lattice relaxation rate measurements in
SrZnVO(PO4)2, a S=1/2 frustrated magnet on a square lattice, are presented. The
temperature (T) dependence of the in-plane correlation length \xi (T) is
derived and it is shown that the overall behaviour is analogous to the one
found for non-frustrated systems but with a reduced spin stiffness. The
temperature dependence of \xi (T) in SrZnVO(PO4)2 is compared to the one of
other frustrated magnets on a square lattice with competing nearest neighbour
(J1) and next-nearest neighbour (J2) exchange couplings and it is shown that
\xi (T) progressively decreases as the ratio J2/J1 approaches the critical
value leading to the suppression of long-range magnetic order. In spite of the
differences in the functional form of \xi (T) found in different vanadates, it
is pointed out that the characteristic energy scale describing spin
correlations in all those compounds appears to scale as |2J2 + J1|.
