Ralfeez - I have J1.5.23 and VM1.1.9 and I got it successfully working. I am doing Paypal (New API) with NO other forms of payment. Paypal Express Checkout DISABLED. First check your settings with mine. I have uploaded two pictures of what I have configured. Obviously I blacked out my API Sig and other private details but you will get the basic idea.
I picked and chose two working fixes from the incredible amount of solutions out there. Before doing the below solutions, IF you have a custom Virtuemart template such as VMJ Color your directories will have that name in them as opposed to "default".
1. The first fix I did was:
Add this line of code -> JHTML::script('joomla.javascript.js','includes/js/');
Here -> /components/com_virtuemart/themes/default/templates/checkou/list_payment_methods.tpl.php
2. Second fix I did was:
Comment out this line echo ps_checkout::list_payment_methods( $payment_method_id );
From -> components/com_virtuemart/themes/default/templates/checkout/
The result is now after step 2 in the checkout process it stops at step 3 and lets you choose Paypal, in my case it is the only choice so the buyer must simply click it. Step 4 redirects them to Paypal where they finish the checkout, my secure header was also present on the Paypal website which is good, meaning the API is working.
Hope this helps someone out, was a pain to finally figure out but thanks to the bits and pieces on the web it finally was solved. Good luck! Any more questions about my setup just ask.
[attachment cleanup by admin]