What are the interactions between Rage and Vaprak’s Rage?

Question

Disclaimer : this is an ultra-specific oddball that will likely never happen to anyone but me. Read only if you have time to kill !

Shapechange is a 9th level spell that allows a player character to assume the form of another creature, while retaining their race/class/other benefits if the new form can physically use them. While the spell is normally restricted to Self-only, it is allegedly possible to make a spell glyph of this spell (requiring access to two 9th level spell slots, such as with a Tome of the Stilled Tongue or a Boon of High Magic) to activate onto a different target than the glyph caster — opinions vary on whether bypassing a self-requirement in this way is possible, but for the purpose of this question, let’s assume that it works.

The Frost Giant Everlasting One has an ability called Vaprak’s Rage, which is similar to, but not identical to a Barbarian’s Rage class feature :

As a bonus action, the giant can enter a rage at the start of its turn. The rage lasts for 1 minute or until the giant is incapacitated. While raging, the giant gains the following benefits: - The giant has advantage on Strength checks and Strength saving throws - When it makes a melee weapon attack, the giant gains a +4 bonus to the damage roll. - The giant has resistance to bludgeoning, piercing, and slashing damage.

Suppose that a Wizard makes a spell glyph of shapechange, and then, that a Barbarian triggers the glyph and assumes the form of a Frost Giant Everlasting One.

Now that our premise has been established (a character with access to Rage and Vaprak’s Rage at the same time), several questions emerge :

1. Can the Shapechanged Barbarian benefit from both Rage and Vaprak’s Rage, gaining both damage increases ? (Not sure whether the “Same Name Effects Stacking” rule applies here)

2. If, for some reason, the Barbarian has access to spellcasting, can he cast spells & concentrate on them while Vaprak-Raging but not regular-raging ?

3. Are his Rage-related class features, such as Frenzy (if the Barbarian is a Berserker) and Relentless Rage, “enabled” while Vaprak-Raging but not regular-raging ?

4. Other interactions I may not have thought about, etc.

Answer 1

The features stack because they do not have the same name

The section from the DMG states:

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap.

- Dungeon Master's Guide (page 252; Combining Games Effects errata)

Additionally there is this section of the Essential's Kit:

Different effects in the game can affect a target at the same time. For example, two different benefits can give you a bonus to your Armor Class. But when two or more effects have the same proper name, only one of them applies while the durations of the effects overlap (a duration is a time span of 1 round or more). The most potent effect - such as the highest bonus - is the one that applies, or the most recent effect applies if the effects are equally potent.

- Essentials Kit (page 9; Effects with the Same Name Don't Stack)

In this case, however, the features do not have the same name. One is called Rage and the other is Vaprak's Rage, thus all of their effects may stack. Note that Vaprak's Rage does not require you to make an attack or be attacked like the regular Barbarian's Rage does. Instead it grants the following benefits:

• The giant has advantage on Strength checks and Strength saving throws
• When it makes a melee weapon attack, the giant gains a +4 bonus to the damage roll.
• The giant has resistance to bludgeoning, piercing, and slashing damage.

The Barbarian's Rage grants the following benefits:

• You have advantage on Strength checks and Strength saving throws.
• When you make a melee weapon attack using Strength, you gain a bonus to the damage roll that increases as you gain levels as a barbarian, as shown in the Rage Damage column of the Barbarian table.
• You have resistance to bludgeoning, piercing, and slashing damage.

The first and last benefits don't actually change anything because having advantage multiple times changes nothing, and you can't benefit from multiple instances of resistance. However, the middle benefits will both apply, granting you at least a +7 to damage rolls and a +8 upon reaching level 16.

Another small difference is that Vaprak's Rage benefits melee weapon attacks that don't use Strength, such as if you made an attack with a finesse weapon for some reason, in this case you would only have a +4 to the damage roll.