Я не знаю, как у вас структурирован массив, но:
Dim text As String = "Hello <<FirstName>> <<LastName>>"
Dim foo() As String
For i As Integer = 0 To UBound(foo)
Dim modifiedText As String = text
modifiedText = Replace(modifiedText, "<<FirstName>>", foo(i,0), 1, -1, vbTextCompare)
modifiedText = Replace(modifiedText, "<<LastName>>", foo(i,1), 1, -1, vbTextCompare)
Console.WriteLine(modifiedText)
Next