I think you may have the wrong mother and this is why you are stuck? There is only one Pieter Vallentgoed born in 1895 to a Johannes Anthonius Vallentgoed on 6 Jan 1895. The birth mother at least here is Maria van der Vijgh.https://www.wiewaswie.nl/en/detail/28082134
Johannes Anthonius Vallentgoed dies 22 Feb 1914 in Spaarndam his wife Maria van der Vijgh is still alive at this time.https://www.wiewaswie.nl/en/detail/27829368
Any other info could be helpful in determining to see if we have the right Vallentgoed. Who is your great grandfather and how is he Johannes Anthonie's half sibling?