The IGI website (
http://tinyurl.com/5kwh ) has a fair list of records for VENUS in Kent.
It includes Thomas, plus Joseph and Elizabeth's marriage, but perhaps thats where you got the info you already have?
It doesn't have any other entries for Joseph anywhere in the country (not just Kent), but then their records are far from complete.
Do you have any clues, however small, that might suggest where Joseph came from, if perhaps it wasn't Kent?
I also found this
Name: Thomas Venus
Gender: Male
Birth Date: 13 Dec 1796
Christening Place: Seven Oaks, Kent, Eng
Father's Name: Joseph Venus
Mother's Name: Elizabeth
Maternal Grandfather's Name: John Turner
Maternal Grandmother's Name: Ann
The original source for this is:
Source Citation: Place: Seven Oaks, Kent, Eng; Collection: Dr. William's Library; Nonconformist Registers; Date Range: 1796 - 1796; Film Number: 815988.
This fits in with the IGI record's comment "Baptist", and Eynsford is very close to Sevenoaks.
So it might be worth checking out the National Archives non-conformist records. Viewable at
http://tinyurl.com/yzaw4lu