Were Viktor and Alexander Corvinus acquaintances before they became immortal?

Timelines suggest not. Alexander was a warlord in the late 4th and early 5th centuries. The Plague occurred in the early 5th century. Based on his appearance alone, basically, his actor, Derek George Jacobi, age at the time of the movie (63), Alexander was in his late 60s when the plague hit. Add 20 to 35 years for Marcus to be born and mature to adult hood (Tony Curran was 37, but lets be generous). Viktor was in his dying years, old age, so lets place him at 70. But he's explicitly placed being born in the 5th to 6th century, by counting back from modern day:

"...Viktor was a haughty, ostentatious Vampire Elder over 1,400 years old." ~ Underworld: Evolution production notes, Underworld: Rise of the Lycans production notes, Underworld: Evolution official site

"Viktor is a haughty, ostentatious Vampire overlord nearly 1,000 years old." ~Underworld: Rise of the Lycans official site

Keep in mind, William wasn't captured by Viktor's vampire army until 1202 AD, the 13th Century, to put in perspective. William was rampaging from 550 up to 700 years. Not knowing how long before Viktor Alexander was made immortal, or his sons were born or turned makes it difficult to pin down, but we know it should be at max 200 years before Viktor was turned.

Essentially, the math puts Viktor as being half to a full century past Alexander.

Timeline:

1. Alexander is born somewhere between 300 and 500 AD.
2. Alexander, Warlord, returns home somewhere between 400 and 500 AD.
3. Alexander is made immortal somewhere between 400 and 500 AD.
4. Everyone else in Alexander's home village dies.
5. William and Marcus are born and bit between 400 and 500 AD.
6. Viktor is born somewhere between 400 and 500 AD.
7. Viktor leads full life, about to die of old age somewhere between 500 and 600 AD.
8. Marcus makes Viktor an offer he can't refuse somewhere between 550 and 600 AD.
9. Marcus likely regrets this a day later.

Timeline aside, we don't know how big of a warlord Alexander or Viktor are. Hungary at the time was full of warlords, Atilla the Hun included:

The Visigoths, Alans, Vandals and most of the Quadi and Marcomanni, however, left this territory around 400, and moved on to western and southern Europe.

The Huns, taking advantage of the departure of the Goths, Quadi, et al., created a significant empire in 423 based in Hungary. In 453 they reached the height of their expansion under the well-known conqueror, Attila the Hun. The empire collapsed in 455, when the Huns were defeated by the neighbouring Germanic tribes (such as the Quadi, Gepidi and Scirii).

The Gepidi (having lived to the east of the upper Tisza river since 260 AD) then moved into the eastern Carpathian Basin in 455. They ceased to exist in 567 when they were defeated by the Lombards and Avars (see below).

The Germanic Ostrogoths inhabited Pannonia, with Rome's consent, between 456 and 471.

No reason to suspect they personally knew each other or even occupied the same region. After all, Viktor took over Castle Corvinus with Marcus by 15th century, instead of his own. Alexander was long gone.