To understand best where the Virginia class is going with the VPMs, and how it will develop into the replacement for the Ohio SSGNs, you have understand how the US Navy got to where it is.
First, the US Navy began installing Vertical launchers on its Flight II Los Angeles class nuclear powered attack submarines, beginning with the USS Providence, SSN-719, which was commissioned in 1985. Thirty-one Los Angeles class submarines were built with this capability. This same type of capability (meaning twelve individual VLS Cells) have been installed on the initial two blocks of the Virginia class submrines.
There are twelve of these on those later class Los Angeles class SSNs, and there are twelve on the first ten Virginia class SSNs. This gives these boats the capability to fire up to twelve Submarine Launched Cruise Missiles (SLCMs) at enemy targets
After this was done with the later LA Class boats and while being built into the intial Virginia Class boats, the US Navy modified four Ohio Class SSBN nuclear ballistic missile submrines to meet its nculear missile treaty obligations. The four modifed boats became SSGNs, nuclear powered guided missile submarines.
The modification took twenty-two of the twenty-four trident missile silos, and placed an insert into those silos that could fire seven Tomahawk SLCMs each. These are called Multiple All-up Round Cannisters (MACs). This allowed for 154 SLCMs for each of those four boats. The other two tubes were modified to be lock-in, lock-out chambers for Navy SEALs.
These modules were designed for furture contigencies in mind. So in addition to being able to launch up to seven Tomahawk SLCM each, they were also designed to accomodate numerous future weapons. These would include the next generation of supersonic or hypersonic cruise missiles, unmanned air vehicles (UAVs), various sensors and weapons for anti-submarine warfare, various sensors for intelligence, surveillance, and reconnaissance missions, countermine warfare payloads such as the Long Term Mine Reconnaissance System (LMRS), other specialized payload canisters such as the broaching universal buoyant launcher (BUBL) or stealthy affordable capsule system (SACS) or the the ADM-160 MALD decoy system.
The four Ohio SSGN boats are going to be retired sometime in the late 202s to early 2030s. The US wants to augment and replace those SSGN boats, but has decided not to creat a seperate class to do so. They intend to lengthen the Virginia Class submarines, starting woith the block V boats in order to give more space for more SLCMs.
Here's how they plan to get there.
Starting with the current Block III build of the USS North Dakota (which was launched in 2013), the twelve indivdual vertical tubes shown above, or going to be replaced on the bow of the vessel with two special Mulitple All-up Round Cannisters. Each of these MACs will be able to fire six tomahawk SLCMs and use the same technology employed for the Ohio SSGNs (meaning the ability to launch all the various sytems that the Ohio MAC was designed to accomodate), but updated and altered for the Virginia Class submarines.
All eight Block III Virginia class SSNs, and all nine Block IV vessels will be built with the new Virginia MACs for firing their SLCMs and other weapons.
Then, beginning in 2019, with the start of conbstruction of the first Block V Virginia Class submarines, the submarine will undergo an even larger modification. Aft of the sail, an approximately 80-100 foot section will be added to the boats. This section will be specifically designed to house four new Multiple Launch Vertical Missile cells called Virginia Payload Modules (VPMs). Each of these modules will allow seven tomahawk SLCMs to be launched (along with the other types of payloads mentioned for the MAC). For of these modules will be added to each of these vessels.
These subs will then be built in some numbers...up to twenty of them. They will be the replacemnt for the Ohio SSGNs, except they will be hybrid vessels that can perform equally well as either a fully capable SSN, nuclear powered attack submarine, or as an SSGN, nuclear powered guided missile submarine carrying forty SLCMs each.
This is how the US Navy has gotten to the Virginia VPM, and how it will be used to replace the Ohio SSGNs. By the time the VPM is active in the early 2020s, the US Navy will have spent 35+ years getting there.
And India started from there - that too decades earlier.
Although not as capable - I found Arihant to be of similar configuration
6000-7000t nuclear powered with torpedoes + Vertical launching tubes. And each VLS can contain multiple missiles.
