As you are aware, the pAncarAtric rite of panca-samskAra establishes a formal link between the initiate and an acharya of the tradition of Ramanuja. The vaikhanasas as stated above do not have such a ritual, and do not formally have a connection with Ramanuja. However, while they are not branded with the insignia of Vishnu, they believe that Vishnu himself comes to the womb in the third month of pregnancy and brands the child with the sankha and cakra. This is known as 'garbha-samskAra' and is dictated once again by the vaikhAnasa texts.
The vaikhAnasa texts are overwhelmingly concerned with the details of temple ritual and largely do not contain philosophy. Most of the philosophical teachings are similar to the pAncarAtra, including a parallel five-fold manifestation of Vishnu. They also have a notion of the 'nishkala' form of Vishnu -- the formless, primeval Vishnu which is perceived only by the highest of yogis and which is beyond even Brahma -- and the 'sakala' form, which is figured, divisible, and emanated. It is in this form that Vishnu responds to devotion and meditation. There is also 'sakala-nishkala' combination of the two, which is found in the sAlagrAma. (These details may also exist in some pAncarAtra texts).
People often confuse vaikhAnasas with Sri Vaishnavas because of their similar outward resemblance. While the vaikhAnasas are strict vaishnavas, they do not revere the Alvars and they do not even follow Ramanuja's Sribhashya the same way we do. Many people also assume that the Tirumalai temple (Tiruvengadam) is a Sri Vaishnava shrine. In fact, the shrine is a vaikhAnasa one with strong Sri Vaishnava association, but is not really a Sri Vaishnava shrine. Due to the influence of Periya Tirumalai Nambi (uncle of Ramanuja), Ramanuja, Tirumalai Anandaan Pillai, the Sri Vaishnava influence increased over the years. The shrine took a more pAncarAtric tilt (more festivals, recitation of Alvar paasurams, etc.) after the utsava mUrti of Lord Ranganatha took refuge along with the Srirangam priests in the 13th century. The cross-pollenation has led to the mixture of vaikhAnasa with Sri Vaishnava touches we have today.
Note that there are no shrines to the Alvars on Tirumalai beyond the dhvaja stambham, and even the shrine to Ramanuja is a rather late one. Apparently the vaikhAnasa Agamas do not permit the installation of idols of human beings inside their temples. The shrine to Ramanuja is a remarkable exception.
Tirumalai is the purest example of a vaikhAnasa temple we have today. There are many other vaikhAnasa temples in Tamil Nadu and elsewhere, (Tiruvallikkeni, and Vaanamaamalai for example) but most of them have been significantly influenced by pAncarAtra practices due to the surrounding Sri Vaishnava community, so much so that the temples are virtually indistinguishable from pAncarAtra temples in style (the rituals may vary somewhat). In addition, many temples that were once vaikhAnasa were converted to pAncarAtra during Ramanuja's days, mostly because of a more liberal ritualistic attitude.
Credits: Mani Vardarajan