Access is fine with just that [VendorID]? Thats surprising. I'd think you'd have to do "VendorID=" & VendorID
Anyways, this should work
=IIF(CheckboxName,DLookUp("VendorPhoneAppCharge","VendorsT",[VendorID]),0)

is this in a query or VBA?

Its in the default value for text box

=IIF(CheckboxName,DLookUp("VendorPhoneAppCharge","VendorsT",[VendorID]),0)  this is basically what I had put in

also used  "VendorID=" & VendorID same results  shows default as 0.00 but when I checked phoneApp yes/no it stays on 0.00

That would only set the default value, if you want to click the checkbox while the records open and make it change youd have to set the value of the field or make the iif statement in a query or control source and you may need to requery it

A default value is *already set* for the current new record before you even click the checkbox, so it can't be changed.  You have to enter a new record, then the *next* new record will have the new default value.  This, obviously, is a broken mechanism, so the default value doesn't work.  Just assign whatever value you want to the control whenever you want it.

I use this method on other parts of database that work fine.  I just can't get this one working so I placed default to 0.00 and need to change the value when value change is needed.

A default value cannot be "dynamic" (i.e. changing over time or via user actions).  For instance, Now() is a "dynamic" expression.  if you set the default value of a date field to =Now(), you open the form at  7/17/24 7pm, and the default value will remain 7/17/24 7pm until you enter a new record.  If you leave the form open for 4 hours, and it will still be defaulted to 7/17/24 7pm, even though it will be 4 hours earlier than the present value of Now().

Dynamic Default

For me, the simplest way is to simply assign whatever value you want during the BeforeInsert event.  E.g.:

     Private Sub Form_BeforeInsert(Cancel As Integer)
         Me.datefield1 = Now()
     End Sub

The above code will assign the up-to-the-second Now() value to the field the moment the user types on the new record.  You can use any logic you need to assign whatever value for the new record, at that moment.  It doesn't need to use the default value feature at all.

The solution was a laugh.  At the new record I hadn't placed in the Vendor yet.  Made a minor change and working