Each of the vassals will likely border other French vassals, with France as an overlord the Dukedoms are safe from attack but they are vulnerable to a demand for annexation. It is not necessary for form an alliance with France to be protected.
Trade and Diplomacy
At the start of the game there is plenty of time and opportunity to expand diplomatically and economically, while ignoring France's pressures of annexation. Thankfully, unlike when a player demands annexation of an AI nation, relationships between AI France and vassal do not go down if the player declines the AI's request. In addition, AI France prefers to use diplomacy to annex her vassals (for at least twenty years), and good starting relationships (150) along with diplomatic skill almost guarantee France will not attack. In fact, Berry's excellent diplomatic skills can even convince regional powers like Hungary to offer a military alliance after expanding into a mere three province minor.
As a one province minor, there will be plenty of time to build up spies and diplomats for use weakening France whenever possible. While France will protect her vassals, she too will put considerable pressure on annexation (game over) and will quickly abandon any vassals who declare independence. Spies and defections is the only reliable way to gain provinces while still a vassal due to the inability to negotiate peace deals, and the nearby landlocked provinces of Provence are an ideal first target. Provence's capital is further south and unlike the other minors nearby, is not protected or guaranteed by France. If Naples declares war in retaliation, Provence/Naples will have no way to attack Berry as it is a landlocked country. Remember however to try to become friendly with neighbouring countries, particularly Burgundy, before declaring independence for potential allies or guarantees. If considering a long term investment in trying to join the HRE, befriending Burgundy and the Emperor is a good investment.
Declaring freedom from France should be no problem assuming an excellent relationship exists between Berry and one of the regional powers, possibly France herself, Burgundy, or both. Military Alliances with France, along with Military Access agreements, ensure a deterrent against France attacking in retaliation due to the cumulative stability hit.
The New World
While landlocked use missions to expand and gain access to the sea. Spies will enable this mission to be completed without drawing French aggression, opening up new doors to the new world, if and only if France is preoccupied and will not be able to defeat the rebel stack.
If possible, try to cut France to pieces using missions to expand through the nation. This will make it harder for France to properly unite, and at the same time, fight against invaders, such as England. Depending on England to take down France is not ideal, however, due to the disadvantages the AI has on sea landings (lack of proper reinforcement until a large stack gets run down on attrition). Instead, wise usage of Berry's diplomatic skills in courting Aragon and Burgundy or the Holy Roman Emperor along with sound financial investments to finance mercenaries can lead to the collapse of France if done early before France successfully annexes the various French Dukdoms.
Alternatively, it is possible to inherit France in a Royal Marriage, but unlikely unless France has fewer provinces, or is force unified under a "Forged Documents" mission.
Joining the HRE can be a route to military power and even the ability to dominate Europe. It is not necessary to be a member of the HRE to be emperor, although being included is beneficial as non members are usually only elected if they are a sizeable regional power and a suitable one does not exist inside the empire (ex.: a large member of a different religion, such as Protestant, will not get votes from smaller Catholic electors), so courting the existing emperor, the next presumptuous emperor, and maintaining a land link to the Empire is preferred.