You say this because you have no clue what the WSL VT system entails and how much time and effort it takes to learn properly. DP spent a brief time with WSL and the results are pretty clear to see. I am not sure why you are getting offended on his behalf?
I don't have a horse in that race. My issue is that you seem to dodge WSL personally saying DP was fit to teach WSLVT. When something like that happens it is the head of the "family" saying the person's knowledge of his art is very extensive. As you say it takes time to learn WSLVT, heck any art. So this is, in essence, an indicator of the time involved.
I think there are much better ways to prepare for with people armed with knives when you are unarmed (if that is your concern) than to train awful wing chun grappling in an attempt to deal with it. It reminds me of the comments where people were trying to convince me that adding grappling to VT is worth doing when bjj, judo and wrestling exist and are easily available in most places.
I think it is inappropriate to make a global statement that all WC lineage has bad grappling.
I agree with your last bit though. To personally try to add something into any existing system, not just WSLVT, and maintain coherence isn't a good idea. However cross training in another art would, imo not be out of the question and as I said, some WC Lineages do have functional Chin Na.
From what I have seen of wing chun grappling I would prefer to rely upon good WSL VT skills if I was unarmed, than to attempt some second rate grappling. But since I am a judo blackbelt and bjj purple in all honestly if someone came at me with a knife I would most likely try to secure the weapon arm, trip them to the ground, and break the arm.
And here you kinda make my point. My point was simple really, just 3 parts.
1. A significant amount of self defense scenarios have someone encountering a weapon.
2.thus if you are training for self defense Chin Na is an advisable skill.
3. Some WC Lineages have function Chin Na, so they should be considered IF #2 is true. If you want to cross train as you have done, that works as well.
Thankfully defending unarmed against weapon attacks has not been a big part of my life so far, whereas I have had a few unarmed altercations. If people were regularly attacking me with knives then I would carry a weapon.
I look at such skills as a "be prepared" thing. Off duty I usually carry weapon(s) of some sort but action beats reaction and sometimes trying to deploy a weapon can put you in a bad spot, you just need to control that weapon bearing limb and do what you have to to disarm the suspect. Sadly where I work violence is not uncommon.
VT addresses an armed assailant through the weapons. A much higher % approach.
As do other Lineages BUT walking around with the pole or butterfly swords is a bit impractical for street self defence.