Dim name As String = "John"
Dim surname As String = "Doe"
Dim age As Integer = 30

output = "I am " + name + " " + surname + " and I have " + Str(age)  + " years"

Or, if you prefer the Python style, create a module with a method (untested, may be prone to typos)

Function PyReplace(extends s as string, Paramarray terms as String) As String
   For q as Integer = 0 to terms.ubound
      s = s.Replace("%s", terms(q))
   next
   Return s
End Function

and call it with

output = "I'am %s %s and I have %s years".PyReplace (name, surname, age)

Age should be a string too. If not, you could extend the method to receive variants and try to cast their stringValue.

These are intended to be implemented in a Module.

Function Substitute(extends source as string, subs() as string) As String
//--------------------------------------------------------------------------------------------------------------------------------
// Purpose : Substitute '{#}' in source for the indexed element in sub()
// Parameters: source - The source string
// sub() - Strings to be substituted
// Returns : Modified string
// Notes : -
//--------------------------------------------------------------------------------------------------------------------------------

dim modString as string = source

for i as int32= 0 to subs.Ubound
modString = modString.ReplaceAll("{"+str(i)+"}", subs(i))
next i

return modString

End Function

Function Substitute(extends source as string, paramarray subs as string) As String
//--------------------------------------------------------------------------------------------------------------------------------
// Purpose : Substitute '{#}' in source for the indexed element in sub()
// Parameters: source - The source string
// subs - Strings to be substituted
// Returns : Modified string
// Notes : -
//--------------------------------------------------------------------------------------------------------------------------------

dim mySubs() as string = subs //Convert the paramarray into an array

return source.Substitute(mySubs)

End Function

See the StringUtils library here: http://www.strout.net/info/coding/rb/intro.html

Coming from Python you will probably appreciate the Slice methods included in ArrayUtils :)