These overprints are simple forgeries made by a collector from Poland. You may easily recognize them as they have Lithuanian postmarks different from those used in Central Lithuania.@qatarstamps
You have acquired a very nice set of Central Lithuania overprints.
Please see the scan below.
Stamps marked with a green dot have genuine Vilnius 1 postmark. You can be 99% sure they have genuine overprints as well.
Those with a red dot have a forged Vilnius postmark that you may recognize by a flattened shape in a place marked with an arrow. However, overprints may be still genuine.
Finally, stamps with a yellow dot are impossible to be checked using this small scan only as they haven’t got any other hints.
I suggest contacting Polish Philatelists union expert Janusz Berbeka whose signs and descriptions on different stamps from this issue are quite popular in Poland. You may find his mail in the list below:http://zgpzf.pl/pliki_mr/lista_ekspertow_3.jpg
Example of his Mark:http://images37.fotosik.pl/1340/d532805d6184bb38.jpg