Here you go, just change fillet radius on whatever you want
Attribute VB_Name = "modFilletLines"
Option Explicit
Sub LineFillet()
Dim setName As String
Dim setObj As AcadSelectionSet
setName = "$Lines$"
On Error GoTo Err_Control
Dim fType(0) As Integer, fData(0)
Dim oSset As AcadSelectionSet
fType(0) = 0: fData(0) = "LINE"
Dim dxfcode As Variant
Dim dxfdata As Variant
dxfcode = fType: dxfdata = fData
Dim setColl As AcadSelectionSets
With ThisDrawing
Set setColl = .SelectionSets
For Each setObj In setColl
If setObj.Name = setName Then
.SelectionSets.Item(setName).Delete
Exit For
End If
Next
Set oSset = .SelectionSets.Add(setName)
End With
oSset.SelectOnScreen dxfcode, dxfdata
If oSset.Count <> 2 Then
MsgBox "Select 2 lines only"
Exit Sub
End If
Dim lnf As AcadLine
Dim lns As AcadLine
Set lnf = oSset.Item(0)
Set lns = oSset.Item(1)
Dim hdlf As String: hdlf = lnf.Handle
Dim hdls As String: hdls = lns.Handle
Dim filrad As Double
filrad = ThisDrawing.GetVariable("filletrad")
ThisDrawing.SetVariable "filletrad", 25#
ThisDrawing.SetVariable "cmdecho", 1
Dim cmd As String
cmd = "(command " & Chr(34) & "_.fillet" & Chr(34) & " (handent " & Chr(34) & hdlf & Chr(34) & ")" & Chr(32) & "(handent " & Chr(34) & hdls & Chr(34) & ")" & Chr(32) & Chr(34) & Chr(34) & Chr(32) & Chr(34) & Chr(34) & ")" & vbCr
ThisDrawing.SendCommand (cmd)
Err_Control:
If Err.Number <> 0 Then
MsgBox Err.Description
End If
End Sub