Same problem here. Opening already generated invoices turns up with error's instead of the .pdf:

<q>
Warning: imagepng() [function.imagepng]: Unable to open '/home/p13762/domains/rams23.eu/public_html/tmp/VM_Facturen/mska_901401932b4376a14ce7c911f8d3f043' for writing: No such file or directory in /home/p13762/domains/rams23.eu/public_html/administrator/components/com_vminvoice/libraries/tcpdf/tcpdf.php on line 8700

Warning: imagepng() [function.imagepng]: Unable to open '/home/p13762/domains/rams23.eu/public_html/tmp/VM_Facturen/mskp_901401932b4376a14ce7c911f8d3f043' for writing: No such file or directory in /home/p13762/domains/rams23.eu/public_html/administrator/components/com_vminvoice/libraries/tcpdf/tcpdf.php on line 8705

Warning: fopen(/home/p13762/domains/rams23.eu/public_html/tmp/VM_Facturen/mska_901401932b4376a14ce7c911f8d3f043) [function.fopen]: failed to open stream: No such file or directory in /home/p13762/domains/rams23.eu/public_html/administrator/components/com_vminvoice/libraries/tcpdf/tcpdf.php on line 8503
TCPDF ERROR: Can't open image file: /home/p13762/domains/rams23.eu/public_html/tmp/VM_Facturen/mska_901401932b4376a14ce7c911f8d3f043
</q>

Version 2.0.2.0 on J2.5.8 after update from 2.0.1.8 (updated because administration costs showed up after updating VM