VB.Net can do this, but I don't believe VBA can.
Two features that might help you simplify this code in other ways are overloaded functions or optional parameters. Here's an example using optional parameters:
Function complicatedFunction(x as Double, Optional param1 as Double = L, Optional param2 as Double = U) As Object
...
End Function
complicatedFunction(x)
However, L and U must be constants for this to work.
FWIW, and in case it turns out that you're really working with a VB.Net dialect, the VB.Net syntax looks like this:
Sub complicatedFunction(x as Double, param1 as Double, param2 as Double)
...
End Sub
Function mainActionHappensHere(L as Double, U as Double ...)
Dim cleaner As Func(Of Double, Object) =
Function(x)
Return complicatedFunction(x, L, U)
End Function
Dim y = cleaner(x) 'Many calls to this function
...
End Function