What's the series number of the episode written by Dan Vebber?
CREATE TABLE "table1_23242933_2" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT MAX("no_in_series") FROM "table1_23242933_2" WHERE "written_by"='Dan Vebber';