I am surprised that you like both the P-10 and the Vandy 5 because they sound very different. The midrange of the P-10 has a kind of ringing tone, very pretty but I feel it is a bit colored. The high frequency of the P-10 is also more airy and more delicate. I have auditioned both and finally bought the Vandy 5 because I really like its midrange.
Putting the sonic quality aside, since I have been using a pair of Model 5 for over 20 months, I would like to share with you some of my long-term experiences.
First of all, since you own the 3A Sig, you probably already knew this. But for the benefits of other readers, I would like to point out that the 5 does not use binding posts. It has a binding block with four slots at the back for bi-wiring. Each slot has a screw in the middle to secure a spade connector. These slots are very narrow; they wont accept anything wider than 1/2 inch. Nowadays, most of the high-end cables are pre-fitted with heavy-duty spades and will not fit the narrow slots. Having your cable re-terminated might be expensive, depending on manufactures policy, some might do it at cost but others might charge hundreds. If you have expensive cables, you might want to find out in advanced if they will fit or how much re-termination will cost.
One of the most praised features of the Model 5 is the 11-band equalizer of the subwoofer amp. It is designed to give you more placement flexibilities and smoother bass. Unfortunately, proper equalizer adjustment is not so easy; it cannot be done by inexperienced hand with simple test tone and a SPL. You need a Real Time Analyzer and you need to know what you are doing. These subs have very stiff suspensions and need hundreds of hours to run-in. It may take many months depending on how hard you drive them. Without proper run-in; the subs cannot be properly adjusted.
Dealer should perform equalizer adjustment with proper equipment free of charge because the service is included in the purchasing price. I must point out that this is a post-sales service, depending on the attitude of the dealer, it might not be easy to get them over to make the adjustment months after the speakers are shipped and paid for, especially if you live some distance away. This is an unfortunate business reality. So if you decided to buy the 5, you might want to negotiate an agreement with the dealer that would allow you to hold back part of the payment (say a thousand dollars) until the equalizer adjustments are made.
Finally, the upper cabinet - the section that houses the tweeter, the mid-range, and the woofer - has a tendency of developing hairline cracks. It was explained to me that the upper cabinet was constructed by gluing 20 some odd pieces of different cutouts together. As it settles down, due to temperature and humidity change, some pieces may expand or contract a little more than the others and may cause the paint to crack.
The cracks do not seem to affect performance and they cannot be seen with the grille on, but they will definitely affect the resell values. So unless you plan to keep them for a very long time, you might want to take this into considerations.
I am sure other speakers all have different idiosyncrasies. Depending on your priority, the ones I listed here may or may not be important to you but I hope they can help with your decision making.
Best of luck