ValidaCFD

Dim CERT_SIS As String = "C:\PRUEBAS\CERTIFICADO.CER"
Dim CerNo As String
Dim CerSAT As System.Security.Cryptography.X509Certificates.X509Certificate
CerSAT = System.Security.Cryptography.X509Certificates.X509Certificate.CreateFromCertFile(CERT_SIS)
CerNo = StrReverse(System.Text.Encoding.ASCII.GetString(CerSAT.GetSerialNumber))

Otros datos útiles del certificado (en base a este ejemplo):

'Para incluir el certificado en el XML.
Dim CertificadoEnBase64 As String = Convert.ToBase64String(CerSAT.GetRawCertData)

'Para validar su vigencia.
If CerSAT.GetExpirationDateString < Date.Now Then
MsgBox("El Certificado está vencido.", MsgBoxStyle.Critical, "Certificado SAT")
End If