Unparalleled Super Saiyan - SS4 Vegeta is a well-balanced unit with a focus on defense. A great boost to both ATK and DEF from his Passive and starting with some natural high stats allow SS4 Vegeta to hit pretty hard while also absorbing most of the damage he takes. Excluding Super Battle Road and some type disadvantage scenarios against particularly powerful Bosses, this unit can pretty much reduce to nothing any damage that comes from a normal attack. An underrated unit due to the heavy focus on damage output in this game and the lack of good new AGL units.
SS4 Vegeta can fit primarily on two teams, Super AGL, under his own Leader Skill, and "Shadow Dragon Saga".
Super AGL On his own mono team SS4 Vegeta is an amazing unit that synergises very well with most of the other units. Being one of the main damage dealers on the team and also a very good tank, SS4 Vegeta is usually kept on both rotations: the first one with either Mysterious Technique - SS3 Goku EZA or Unparalleled Golden Ki -Super Vegito and the other one with Budding Warrior - Goten (Kid) & Trunks (Kid).
"Shadow Dragon Saga" On this Category team SS4 Vegeta receives a 150% boost to all stats, a well-rounded boost that makes him perform just slightly better than on his own team unfortunately. This is also because the Category has very few units and none of them can activate SS4 Vegeta's strongest links, "Saiyan Pride" and "Saiyan Roar", meaning he is missing on 40% ATK potential.
Unlike Ultimate Super Saiyan - SS4 Goku that was released with him, SS4 Vegeta doesn't have any raising mechanic on his Super Attack, he instead greatly lowers the enemy's DEF, which is not relevant in most cases.
SS4 Vegeta is the 120% leader for Super AGL, a very good element despite the fact that it hasn't received much love lately in the form of new units.
SS4 Vegeta's Passive gives him a 120% boost to both ATK and DEF, a pretty big increase on top of his already naturally high stats, and a 30% chance to counter a Super Attack, the only damage he would not be able to mitigate that well.
"Shocking Speed" is a very important Ki link for a Super AGL unit, a lot of the top-tier units have it (one of the few the exceptions is Mysterious Technique - SS3 Goku EZA), so it's definitely good that SS4 Vegeta has it. "Prepared for Battle" is not as relevant anymore unless you are still running Unparalleled Golden Ki - Super Vegito. "Super Saiyan" and "Fierce Battle" are standard ATK links on most Super type units, but "Saiyan Pride" and "Saiyan Roar", despite being incredibly powerful links, are have active at the same time unless you are bringing specific unit for that purpose like Explicit Tactics - Toa (Great Ape), that would still lack "Fierce Battle" and "Super Saiyan", not a great trade-off.
Super Vegito and SS4 Vegeta link very well together in terms of Ki with "Shocking Speed" and "Prepared for Battle", they activate "Super Saiyan" and "Fierce Battle" for each other and if Super Vegito Super Attacks in the first slot he will further increase SS4 Vegeta's ATK by 30%. 25-55% more ATK and +4 Ki make these two a powerful rotation both defensively and offensively that is also really consistent.
Bardock is a great unit that gives 30% ATK to his own element and activates "Saiyan Pride" for SS4 Vegeta for a total of 45% more ATK and 4 Ki, 2 from his Passive and 2 from "Prepared for Battle". Unlike Toa (Great Ape), Bardock can still function well with the rest of team thanks to his support Passive and can also activate "Shattering the Limit" for Goten (Kid) & Trunks (Kid).
Another great support unit that adds 2 Ki and 25% to ATK and DEF to his element. SS2 Gohan unfortunately lacks "Fierce Battle", but does have "Super Saiyan" and "Prepared for Battle", giving SS4 Vegeta 35% ATK and 4 Ki in total.
Pan (GT) doesn't have any particular ATK link for SS4 Vegeta to benefit from, but her Passive alone gives him 3 Ki and 40% to both ATK and DEF while also sharing "GT" and "Shocking Speed" with him for a total of 7 Ki, meaning SS4 Vegeta linked up with Pan (GT) is going to Super Attack automatically. Pan (GT) raises allies' ATK when she Super Attacks, but since she is not realistically going to be in the first slot of the rotation, that is hardly going to matter.
Since the Category has so little units there is not much of choice in terms of units to pair with SS4 Vegeta. Activating "Super Saiyan" and "Fierce Battle" for 25% ATK and "GT" and "Shocking Speed" for 4 Ki while also debuffing Extreme type enemies' ATK by 20% increasing SS4 Vegeta's tanking capabilities, makes SS4 Gogeta the best partner for SS4 Vegeta on this team.
The standard lv.15 Criticals and lv.11 Additionals is fine with this unit, SS4 Vegeta certainly doesn't need dodges. Investment on his Potential System to increase his base stats will exponentially increase his ability to tank and mitigate damage overall.
Should I use Elder Kais/Grand Kais?
SS4 Vegeta is not currently farmable, but he is totally worth 9 Elder Kais/Grand Kais: he is probably your best choice after Budding Warrior - Goten (Kid) & Trunks (Kid) to invest your AGL Grand Kais on.