Вероятно, вы хотите сделать так.
Sub DoSerialisation()
Dim objLevel = New cls_JSON_Level
Dim objTags = New List(Of cls_JSON_Tags)
Dim objSub = New List(Of cls_JSON_Sub_Tags)
Dim strJSONOUT As String
objSub.Add(New cls_JSON_Sub_Tags With {.number = "B8888888888", .type = 0})
objSub.Add(New cls_JSON_Sub_Tags With {.number = "77777777777", .type = 1})
objSub.Add(New cls_JSON_Sub_Tags With {.number = "66666666666", .type = 2})
objTags.Add(New cls_JSON_Tags With {
.stock_uuid = "aabrjs-123768-kfh897-klasd123",
.number = "78621398912",
.last_seen = Now(),
.subsidiary_tags = objSub.ToArray})
objSub.Clear()
objSub.Add(New cls_JSON_Sub_Tags With {.number = "66666666666", .type = 0})
objSub.Add(New cls_JSON_Sub_Tags With {.number = "55555555555", .type = 1})
objSub.Add(New cls_JSON_Sub_Tags With {.number = "44444444444", .type = 2})
objTags.Add(New cls_JSON_Tags With {
.stock_uuid = "bfgbd189-9ju8un17-sdf01",
.number = "78621398912",
.last_seen = Now(),
.subsidiary_tags = objSub.ToArray})
objLevel.uuid = "8721-asd9012-asdk871-991klkj-910jlk"
objLevel.tags = objTags.ToArray
strJSONOUT = Newtonsoft.Json.JsonConvert.SerializeObject(objLevel, Formatting.Indented)
End Sub