The subliminal messaging Biden communicated was Ryan is a liar.
This was made most clear when Ryan failed to provide the specifics of how much Romney's tax plan would cost and how it would be paid for.
Romney stipulated in the first debate that his tax plan would:
1. not lower taxes for the rich
2. not increase the federal deficit
3. not require any increase in taxes for anyone
Shouldn't such a miracle plan be easy to pencil out? During the Obama/Romney debate, Romney offered this proof of the soundness of his 20% across the board tax cut plan:
"MR. ROMNEY: So — so if — if the tax plan he described were a tax plan I was asked to support, I'd say absolutely not. I'm not looking for a $5 trillion tax cut. What I've said is I won't put in place a tax cut that adds to the deficit. That's part one. So there's no economist can say Mitt Romney's tax plan adds 5 trillion (dollars) if I say I will not add to the deficit with my tax plan.
Number two, I will not reduce the share paid by high-income individuals. I — I know that you and your running mate keep saying that, and I know it's a popular things to say with a lot of people, but it's just not the case. Look, I got five boys. I'm used to people saying something that's not always true, but just keep on repeating it and ultimately hoping I'll believe it — (scattered laughter) — but that — that is not the case, all right? I will not reduce the taxes paid by high-income Americans.
And number three, I will not, under any circumstances, raise taxes on middle-income families. I will lower taxes on middle-income families. Now, you cite a study. There are six other studies that looked at the study you describe and say it's completely wrong. I saw a study that came out today that said you're going to raise taxes by 3(,000 dollars) to $4,000 on — on middle-income families. There are all these studies out there."
http://www.npr.org/2012/10/03/162258551/transcript-first-obama-romney-presidential-debate
To sum up Romney's arguments defending his tax plan: his plan won't lower taxes for the rich, add to the deficit or raise taxes on the middle class because Romney says so.