According to Wikipedia, sodium orthovanadate is prepared by mixing vanadium(V) oxide in a solution of sodium hydroxide:
$$\ce{V2O5 + 6 NaOH → 2 Na3VO4 + 3 H2O}$$
But, there is a different reaction scheme as mentioned in this site:
Sodium Orthovanadate, $\ce{Na3VO4.12H2O}$, is the orthovanadate most
frequently met with. It is readily obtained by adding excess of
caustic soda to a solution of sodium pyrovanadate:
$$\ce{Na4V2O7 + 2NaOH <=> 2Na3VO4 + H2O}$$
It can be conveniently crystallised from caustic soda solutions, as it
is less soluble than in water. It forms hexagonal prisms isomorphous
with the corresponding phosphate and arsenate [...] The anhydrous
salt, $\ce{Na3VO4}$, melts at 850°-866° C. It can be prepared by
fusing sodium carbonate and vanadium pentoxide in the required
molecular proportions. Extraction of the product with water and
precipitation with alcohol gives colourless needle-shaped crystals of
a hydrate containing sixteen molecules of water, $\ce{Na3VO4.16H2O}$.
The hepta- and octa-hydrates, $\ce{Na3VO4.7H2O}$ and
$\ce{Na3VO4.8H2O}$, have also been prepared.
By dissolving vanadium pentoxide in a large excess of caustic soda,
Ditte obtained two crystalline vanadates which contained a larger
proportion of the basic oxide than is present in the orthovanadate,
and to which he gave the formulae $\ce{4Na2O.V2O5.30H2O}$ and
$\ce{4Na2O.V2O5.26H2O}$.