Hi and Welcome,
The pen makes more sense, since you can use up all the insulin, with the vial, for a kitty on a 'regular' dose you will end up wasting a lot of insulin since it will go off before you use it all up, so it works out more expensive in the end.
It's easy to use the pen and draw insulin from it with a syringe, as mentioned above, do not, push air into the pen, it's not advisable.
Do read the stickies at the top of the forum, and ask questions, there's no such thing as a silly question. Have a look at the different dosing methods that we use and see which one is the best fit for you and your boy. We followed decided to follow Tight Regulation, since this was the method that gives kitty the best chance of achieving regulation quickly, which in turn increases the chances of achieving remission.
I would recommend that you make the swap at a time where you can monitor the first few cycles, sometimes you can get a strong response to a switch.
As he's already on vetsulin your starting dose should be based on that, looking at the spread sheet, there's not a lot of data to go on, but 2.5u of vetsulin he was getting to 74 on a pet meter, so I would probably take the dose back a notch to give yourself some breathing room, perhaps to 2u or I might even err to 1.5u, given that there is not much data, and I am suspicious that you may have missed some lower numbers on the 2u.
If you are not planning the change for a few days/cycles yet, getting a little more data on what he is doing on the 2.5u of vetsulin may make the decision easier/safer, you would need to get PS test , morning and night, and try and determine the nadirs over the next few cycles, by getting some midcycle tests between +2 and +6
One thing that does worry me, looking at the ss is that you are shooting blind. Are you having difficulty testing? Shooting blind can be very risky, and looking at the numbers I am suspicious that he may be seeing numbers significantly lower than that 74 on the 2.5u. That pink last night makes me think he dropped lower than normal earlier that day and then bounced.
Hopefully with Lantus you will get a gentler drop in BG, your guy does seem to like to be a bit dramatic, or it may be that the current vetsulin dose is a bit high for him?????