ST_AsMVTGeom AND table name as parameter in pl/psql

I'm trying to develop a plpgsql function that would extract mapbox vector
tiles from a postgresql/post gis database. The database has multiple
geospatial tables, so I want the function to be able to take a table name
as a parameter.

I've gotten the function to work using hard coded table names. Discussion
on functions with table name as a parameter say I need to use EXECUTE
</messages/by-id/20011210134736.D70079-100000@megazone23.bigpanda.com&gt;.
However, when I try and use EXECUTE in the mvtgeom AS section of the script
I get a syntax error on EXECUTE:

CREATE OR REPLACE
FUNCTION public.region3_sim_storms_pg(z integer, x integer, y integer,
stormtable text)
RETURNS bytea
AS $$
DECLARE
result bytea;
BEGIN
WITH
bounds AS (
SELECT ST_TileEnvelope(z, x, y) AS geom
),
mvtgeom AS (
EXECUTE format(
'SELECT ST_AsMVTGeom(ST_Transform(G.geom, 3857), bounds.geom, 4096,
256, true) AS geom, node, bathymetry
FROM %s AS G, bounds
WHERE ST_Intersects(G.geom, ST_Transform(bounds.geom, 4326))',
quote_ident(stormtable)
)
)
SELECT ST_AsMVT(mvtgeom, 'public.region3_sim_storms_pg')
INTO result FROM mvtgeom;
RETURN result;
END
$$
LANGUAGE 'plpgsql'
STABLE
PARALLEL SAFE;

COMMENT ON FUNCTION public.region3_sim_storms_pg IS 'Given a tile address,
storm name query database.';

Past discussion on this topic says this error occurs because the function
is not being recognized as a plpgsql function
<https://stackoverflow.com/questions/6861374/postgresql-syntax-error-when-using-execute-in-function&gt;,
but I have LANGUAGE 'plpgsql' specified. The only thing I can think of is
that things are reverted back to SQL in the "mvtgeom AS (" portion of the
script. Is there a way to use EXECUTE in the "mvtgeom AS (" portion of the
script?

Thanks
Jim