Question: The Roman commander Julius Ceasar referred to Germania as the vassal state ruled by who?

Context paragraphs:
History of Germany: The concept of Germany as a distinct region in central Europe can be traced to Roman commander Julius Caesar, who referred to the unconquered area east of the Rhine as "Germania", thus distinguishing it from Gaul (France), which he had conquered.  The victory of the Germanic tribes in the Battle of the Teutoburg Forest (AD 9) prevented annexation by the Roman Empire, although the Roman provinces of Germania Superior and Germania Inferior were established along the Rhine.  Following the Fall of the Western Roman Empire, the Franks conquered the other West Germanic tribes.  When the Frankish Empire was divided among Charlemagne's heirs in 843, the eastern part became East Francia.  In 962, Otto I became the first emperor of the Holy Roman Empire, the medieval German state.

Principality of Upper Hungary: The Principality of Upper Hungary (Hungarian: "Felső-Magyarországi Fejedelemség"; Turkish: "Orta Macar", "Middle Hungary") was a short lived Ottoman vassal state ruled by Imre Thököly.  It was established on 19 November 1682.  The polity agreed to pay 20,000 gold to the Ottomans annually.  In 1685 Thököly was defeated at Eperjes (present-day Prešov) and the Turks imprisoned him because of his previous negotiations with Leopold therefore his realm ceased to exist.

Soran Emirate: Soran was a Kurdish emirate based in the geographic region of Kurdistan, specifically in what is today known as Iraqi (i.e. southern) Kurdistan.  The emirate presumably gained its full independence from the Ottoman Empire shortly after it was captured from Safavid control, in the 1530s, but was later reincorporated into the Ottoman Empire as a semi-autonomous vassal state.  After serving the empire as a semi-autonomous vassal state for the next couple of hundred years, the emirate slowly gained full independence for a second time, during the late 1700s and early 1800s, but was eventually subdued by Ottoman troops in 1835.  Its capital for most of that time was the city of Rawandiz; however, much of the wealthy nobility were staying at the town of Shaqlawa.

Burmese–Siamese War (1568–70): The Burmese–Siamese War (1568–1570) was a military conflict fought between the Kingdom of Ayutthaya (Siam) and the Kingdom of Burma.  The war began in 1568 when Ayutthaya unsuccessfully attacked Phitsanulok, a Burmese vassal state.  The event was followed by a Burmese intervention which resulted in the 2 August 1569 defeat of Ayutthaya, which became a Burmese vassal state.  Burma then moved towards Lan Xang, occupying the country for a short period of time until retreating in 1570.

Kingdom of Westphalia: The Kingdom of Westphalia was a kingdom in Germany, with a population of 2.6 million, that existed from 1807 to 1813.  It included territory in Hesse and other parts of present-day Germany.  While formally independent, it was a vassal state of the First French Empire and was ruled by Napoleon's brother Jérôme Bonaparte.  It was named after Westphalia, but this was a misnomer since the kingdom had little territory in common with that area; rather the kingdom mostly covered territory formerly known as Eastphalia.

Fabius Valens: Fabius Valens of Anagnia (d. 69) was a Roman commander favoured by Nero.  In 69 he was commander of Legio I "Germanica" based in Germania Inferior.  When the troops refused to endorse the new emperor Galba after Nero's death, he had them proclaim Vitellius, the governor of Germania Inferior, as emperor.

Invasion of Ryukyu: The invasion of Ryukyu (琉球侵攻 , Ryūkyū Shinkō ) by forces of the Japanese feudal domain of Satsuma took place from March to May of 1609, and marked the beginning of the Ryukyu Kingdom's status as a vassal state under Satsuma.  The invasion force was met with stiff resistance from the Ryukyuan military on all but one island during the campaign.  Ryukyu would remain a vassal state under Satsuma, alongside its already long-established tributary relationship with China, until it was formally annexed by Japan in 1879 as Okinawa Prefecture.

Vassal state: A vassal state is any state that is subordinate to another.  The vassal in these cases is the ruler, rather than the state itself.  Being a vassal most commonly implies providing military assistance to the dominant state when requested to do so; it sometimes implies paying tribute, but a state which does so is better described as a tributary state.  In simpler terms the vassal state would have to provide military power to the dominant state.  Today, more common terms are puppet state, protectorate or associated state.

Postumus: Marcus Cassianius Latinius Postumus was a Roman commander of provincial origin who ruled as emperor in the west.  The Roman army in Gaul threw off its allegiance to Gallienus around the year 260, and Postumus assumed the title and powers of emperor in the provinces of Gaul, Germania, Britannia and Hispania, thereby founding what scholars have dubbed the Gallic Empire.  He ruled for the better part of ten years before he was murdered by his own troops.

Kayin Ba: Kayin Ba (Burmese: ကရင်ဘ , ] ; 1253–1342) was governor of Toungoo (Taungoo) from 1325 to 1342.  Prior to his governorship, the commoner had served all the rulers of the frontier state since its creation in 1279, eventually rising to mayor of Toungoo by the 1310s.  Taking advantage of the rivalry between King Uzana I and Crown Prince Kyawswa I at Toungoo's overlord Pinya, he seized the governorship in 1325 by assassinating Gov. Saw Hnit.  While he remained a nominal vassal of Pinya, Ba ruled the region like a sovereign ruler for the next 17 years, transforming the hitherto minor vassal state into a regional power.

Supporting paragraphs: Kingdom of Westphalia History of Germany
Answer: Napoleon's brother Jérôme Bonaparte